Fix example script (#320)

* Update input file script and comment it

* Update example script

* Debugging time

* Example script actually works again

* Update README to be a bit more helpful

* Tidy up example input file to remove development variables

* Run example script as part of the CI

* OK MATLAB, stop being a pain

* Fine MATLAB, you win

* Attempt to beat matlab v2

* Nope, MATLAB wins. No CI example script

* MacOS works. And ifmac has to go first 🙃.

* Windows: the whole path seems least of all evils, and the least likely to break.

* Update examples/arc_01/run_pstd_bscan.m

---------

Co-authored-by: Sam Cunliffe <samcunliffe@users.noreply.github.com>
Co-authored-by: Sam Cunliffe <s.cunliffe@ucl.ac.uk>

README.md

@@ -75,6 +75,8 @@ You can run TDMS either directly or from a MATLAB script.
 For beginners, we recommend starting with the demonstration MATLAB script, which you can find in the `examples/arc_01` directory.
 Move into this directory, launch MATLAB, and run the MATLAB script [`run_pstd_bscan.m`](https://github.com/UCL/TDMS/blob/main/examples/arc_01/run_pstd_bscan.m).
 This script will generate the input to TDMS, run TDMS, and display sample output.
+It also commentates on what it is doing, so you can follow along with what is being setup and created at each stage.
+We have also annotated the input file [`arc_01_example_input.m`](https://github.com/UCL/TDMS/blob/main/examples/arc_01/arc_01_example_input.m) that this script passes to `iteratefdtd_matrix.m`.
 
 <details>
 <summary>Troubleshooting</summary>

examples/arc_01/arc_01_example_input.m

@@ -0,0 +1,160 @@
+%% arc_01_example_input.m
+% An example input file that can be passed to iteratefdtd_matrix to create
+% an input file, which in turn may be passed to the tdms executable to
+% perform the simulation.
+% Section numbers (3.2.X) correspond to those provided in the PDF
+% documentation.
+
+%% Characteristic quantities
+% These values are not read by iteratefdtd_matrix, but define
+% characteristic scales for various quantities in the simulation.
+
+% A characteristic wavelength from which we will derive other length-related input quantities,
+% to pass to iteratefdtd_matrix and thus to tdms
+lambda = 1300e-9;
+
+
+%% Input flags to tdms executable
+
+% Whether to use bandlimited interpolation over cubic interpolation
+use_bli = 0;
+% Whether to use the PSTD method over the FDTD method
+use_pstd = 1;
+
+%% 3.2.1 Grid
+
+% Specifies the dimensionality of the simulation. IE whether we are
+% simulating a full electromagnetic field ('3'), or just the 'TE' or 'TM'
+% modes.
+dimension = '3';
+
+% Define the computational grid size, by giving the number of Yee cells along each coordinate axis.
+% With J = 0, we define a 2D simulation rather than a 3D simulation.
+I = 256;
+J = 0;
+K = 256;
+
+% An array of length N_l, where N_l is the number of layers in the
+% multilayer structure to be simulated.
+% The i-th element is the z-index of the Yee cell at which an interface
+% between the i-th and (i+1)-th layer occurs.
+% If set to an empty vector [], the medium is assumed homogeneous.
+multilayer = [];
+% The remaining properties are either N_l-length vectors whose i-th element
+% defines the material property of the i-th layer, or scalars in the event
+% that we have a homogeneous medium.
+epsr = 1.35^2;  % Relative permittivity of the layers
+mur = 1;        % Relative permeability of the layers
+
+% Size of the Yee cell in metres, in each of the coordinate directions.
+delta = struct();
+delta.x = lambda/4;
+delta.y = lambda/4;
+delta.z = lambda/4;
+
+%% 3.2.4 FDTD specific
+% This section appears first as we will be setting f_an in 3.2.2 based on
+% the simulation timestep interval.
+
+% Courant time step
+dt = 2/sqrt(2)/pi*delta.x/(3e8/1.35)*.95;
+
+% Number of time steps to perform
+Nt = 500;
+
+%% 3.2.2 Source
+
+% Whether we are using a compact source condition,
+% https://github.com/UCL/TDMS/issues/259
+compactsource = 1;
+
+% Frequency in Hertz of the incident EM field
+f_an = asin( 2*pi/1300e-9*2.997924580105029e+08*dt/2)/(pi*dt);
+
+% Define the planes where the incident waveforms are introduced.
+% The variable interface has 6 members; I0, I1, J0, J1, K0, and K1, which are 1x2 vectors.
+% The {I,J,K} indicates which coordinate direction {x,y,z} (respectively)
+% the plane is perpendicular to.
+% The {0,1} indicate which plane appears {earlier,later} in the computational grid.
+% For each plane, the first entry is the constant {x,y,z} index of all Yee
+% cells within the plane.
+% The second entry is a boolean indicating whether or not an incident field is to be
+% introduced at that particular plane.
+interface.I0 = [5 0];   % Plane containing Yee cells w/ index of the form (5, j, k), at which NO incident field is applied.
+interface.I1 = [I-5 0];
+interface.J0 = [5 0];
+interface.J1 = [J-5 0];
+interface.K0 = [10 1];  % Plane containing Yee cells w/ index of the form (i, j, 10), at which an incident field is applied.
+interface.K1 = [K-5 0];
+
+% Function names (present in the MATLAB path) to use to generate the
+% source-field on the incident planes.
+% Whether one, both, or neither name should be provided depends on the
+% manner in which you choose to define the source terms; through
+% compactsource and a callable function, or a time-domain field in a pre-made .mat file.
+% In this example, we call the efield_gauss_base function which creates a
+% Gaussian E-field on the incident planes.
+efname = 'efield_gauss_base';
+hfname = '';
+
+% The index of the Yee cell which fixes the origin of the Cartesian coordinate system of the grid.
+% The incident field is defined relative to this coordinate system.
+illorigin = [floor(I/2) floor(J/2) floor(K/2)];
+% Sets the z-coordinate for the origin of the illorigin cell.
+z_launch = 0;
+
+% Spectral width of the modulating pulse of the incident field, in metres
+wavelengthwidth = 120e-9;
+
+%% 3.2.3 Simulation Type
+
+% Specifies the type of source being used ('steadystate' or 'pulsed'). See section 3.2.3 of the
+% documentation PDF.
+sourcemode = 'pulsed';
+
+% Defines the run mode of the simulation, if being run from MATLAB.
+% If tdms is being run at the command-line, these values are ignored.
+% 'analyse' : Sub-results can be saved using the statements in outputs_array
+% 'complete': Only final results will be saved
+runmode = 'complete';
+
+%% 3.2.5 Output
+
+% Not used as run mode is complete.
+% If running TDMS from MATLAB, variables can be extracted from individual timesteps via these commands
+outputs_array ={};
+
+% Boolean indicating whether to extract phasors in the volume of the grid
+exphasorsvolume = 1;
+
+% Boolean indicating whether to extract phasors over a user-defined surface
+exphasorssurface = 0;
+% Specifies the user-defined surface to extract phasors over.
+% phasorsurface has the form [I0 I1 J0 J1 K0 K1], which defines the
+% extremes of a cuboid, whose surface will have phasors extracted over.
+% These quantities are in the interior coordinate system.
+phasorsurface = [5 I-5 1 1 20 K-5];
+
+% Whether to extract detector signals that have been acquired during the
+% simulation.
+exdetintegral = 0;
+
+%% 3.2.6 Perfectly Matched Layer (PML)
+
+% The order of the PML conductivity profile curve
+n = 4;
+
+% Maximum reflection at PML
+R0 = 1e-7;
+
+% Parameter mimicing the conductivity profile in the PML, applicable to dispersive materials only.
+% See the documentation PDF, section 3.2.6.
+kappa_max = 1;
+
+% Number of PML cells in each direction.
+Dxl = 10;
+Dxu = 10;
+Dyl = 0;
+Dyu = 0;
+Dzl = 10;
+Dzu = 10;