Add with_units_base feature

[GabrielKS]

Closes #1229 by adding a new exported function with_units_base that serves as a "context manager" to execute a block of code as if the system's units base were set to a particular value.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.31%. Comparing base (2d4b336) to head (6e8e2f2).
Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1232      +/-   ##
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+ Coverage   84.29%   84.31%   +0.02%     
==========================================
  Files         181      181              
  Lines        8391     8399       +8     
==========================================
+ Hits         7073     7082       +9     
+ Misses       1318     1317       -1     

Flag	Coverage Δ
unittests	84.31% <100.00%> (+0.02%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
src/PowerSystems.jl	0.00% <ø> (ø)
src/base.jl	88.98% <100.00%> (+0.10%)	⬆️

... and 1 file with indirect coverage changes

[daniel-thom]

LGTM

[kdayday]

Could you add an example at the end of this tutorial section showing with_units_base? https://nrel-sienna.github.io/PowerSystems.jl/stable/tutorials/creating_system/#Changing-System-Per-Unit-Settings
and mention in https://nrel-sienna.github.io/PowerSystems.jl/stable/explanation/per_unit/ with the other functions (halfway down). Thanks!

[GabrielKS]

@kdayday fixed the docstrings and added to that tutorial and that explanation page — how do those look?

[jd-lara]

@GabrielKS can you rebase this branch to update it?

[GabrielKS]

@GabrielKS can you rebase this branch to update it?

Done