Refactor closure so can use other forms

[cmacmackin]

This refactor the current implementation of the Braginskii closure so that it will be easier to swap it out for alternative formulations. A number of new unit tests were added before the actual refactoring began, to ensure nothing is broken.

[codecov-commenter]

Codecov Report

:x: Patch coverage is 54.72155% with 187 lines in your changes missing coverage. Please review. :white_check_mark: Project coverage is 26.39%. Comparing base (0e601f7) to head (f05be18).

Files with missing lines	Patch %	Lines
src/braginskii_conduction.cxx	52.56%	61 Missing and 13 partials :warning:
src/braginskii_ion_viscosity.cxx	48.57%	34 Missing and 2 partials :warning:
src/braginskii_friction.cxx	58.02%	32 Missing and 2 partials :warning:
src/braginskii_heat_exchange.cxx	55.93%	26 Missing :warning:
src/braginskii_collisions.cxx	78.12%	5 Missing and 2 partials :warning:
src/evolve_energy.cxx	0.00%	4 Missing :warning:
src/braginskii_thermal_force.cxx	25.00%	0 Missing and 3 partials :warning:
src/evolve_pressure.cxx	0.00%	2 Missing :warning:
src/braginskii_electron_viscosity.cxx	75.00%	1 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #399      +/-   ##
==========================================
+ Coverage   22.50%   26.39%   +3.89%     
==========================================
  Files          87       90       +3     
  Lines        8071     8107      +36     
  Branches     1146     1132      -14     
==========================================
+ Hits         1816     2140     +324     
+ Misses       6085     5748     -337     
- Partials      170      219      +49     

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[cmacmackin]

@mikekryjak Here is the slide from my ECW presentation which explains what this PR is doing (in very high-level terms). The full presentation is too large to attach to a GitHub comment, but I can share it with you if you like.

The other thing this PR does is add some more unit tests, but hopefully those are self-explanatory.