Release 3.2.0

This provides improvements in the management of Birks' saturation in
ionized materials, a new reader for the HEPEVT kinematics input
format, and various other accumulated bug fixes.

Notable changes in 3.2.0

• Add a new kinematics generator to accept HEPEVT format files. The
  generator input file is set using
  "/generator/kinematics/hepevt/input [file]" and selected using
  "/generator/kinematics/set hepevt". The reader checks the syntax of
  the input file and adds (hopefully) useful error messages. The
  current behavior is to terminate when an input error is found. It is
  currently very picky and only accepts an input file that follows the write
  format described in the G4HEPEvtInterface.cc file. Future release are
  anticipated to become more general.
• Update the detector construction so that it look for BIRKSCONSTANT
  in the material properties table and copies the value into the
  material ionization properties. This works around a missing feature
  in GDML where certain material description values cannot be set. In
  GDML, add the property "BIRKSCONSTANT" with the value in units of
  "mm/MeV" and edep-sim will "take care of the rest".
• Add /edep/material/birksConstant to allow the Birks constant to
  be set for any material. This must be used before the /edep/update
  macro command. It accepts a material name, the value of the Birks
  constant, and the units for the birks constant. For example:
  /edep/material/birksConstant Scintillator 0.12 mm/MeV
• Add an example of generating a GDML file using GeGeDe. This shows
  how to build a gdml file that sets the auxvalues needed by edep-sim,
  and can be used as a starting point for a user geometry.
• Improvement: Add a new include file (EDepSimUnits.h) for use with
  the edepsim_io library. The output of EDepSim is in the same units
  as Geant4 (i.e. the CLHEP units). Adding this include file means
  that the CLHEP SystemOfUnits.h file is not required.
• Improvements: The GDML validations have been cleaned up. The output
  still should be inspected using ROOT and edep-disp, but the tests
  are simplified to make it easier to interpret the output. This is
  mostly intended as an edep-sim developer (pre-)release tool.
• Bug fix: Make sure that the default geometry builds without
  overlaps, and add a validation to catch when it does not.
• Bug fix: Add error traps in EDepSimRooTrackerKinemGenerator to try
  and catch malformed rooTracker files before processing. This tries
  to prevent a silent failure.
• Remove dead test files that are left over from old, and unsupported
  methods of reading the edep-sim output.
• Bug fix: Fix some compilation warnings. The compilation should be
  warning free.
• New option: Add a cmake definition to disable the compilation of
  NEST. The default is to include NEST. Even if NEST is not included
  in the executable, ionization in argon cribs from it's
  implementation, so the NEST paper should be cited.