Add muon-catalyzed fusion inp:: skeleton and basic import

[stognini]

This PR adds the skeleton for the input structures needed to load muon-catalyzed fusion into the code:

• Add MucfPhysics option to input::Problem::Physics
• Add option to toggle on/off muCF physics in GeantPhysicsOptions
• Load hardcoded default data in GeantImporter when Geant4's muonic atom capture process is present
• Initialize G4MuonMinusAtomicCapture in SupportedEmStandardPhysics::add_mu_processes
• Add a new small GDML test geometry (mucf-box.gdml) which is simply a box with a mixture of hydrogen, deuterium, and tritium for testing
• Add new test to GeantImporter.test
• Draft initial documentation

[github-actions[bot]]

Test summary

 5 702 files   9 188 suites   18m 2s :stopwatch:  2 087 tests  2 061 :white_check_mark:  26 :zzz: 0 :x: 31 937 runs  31 789 :white_check_mark: 148 :zzz: 0 :x:

Results for commit 0afdfe93.

:recycle: This comment has been updated with latest results.

[codecov[bot]]

Codecov Report

:x: Patch coverage is 47.19101% with 47 lines in your changes missing coverage. Please review. :white_check_mark: Project coverage is 84.80%. Comparing base (8d0d904) to head (0afdfe9).

Files with missing lines	Patch %	Lines
src/celeritas/inp/Physics.hh	0.00%	17 Missing :warning:
src/celeritas/ext/GeantPhysicsOptions.hh	66.66%	0 Missing and 13 partials :warning:
src/celeritas/g4/SupportedEmStandardPhysics.cc	59.09%	5 Missing and 4 partials :warning:
src/celeritas/ext/GeantImporter.cc	16.66%	5 Missing :warning:
src/celeritas/inp/PhysicsProcess.hh	0.00%	3 Missing :warning:

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2125      +/-   ##
===========================================
- Coverage    84.86%   84.80%   -0.07%     
===========================================
  Files         1255     1256       +1     
  Lines        44186    44233      +47     
  Branches     16504    16534      +30     
===========================================
+ Hits         37499    37512      +13     
- Misses        4726     4758      +32     
- Partials      1961     1963       +2     

Files with missing lines	Coverage Δ
src/celeritas/Types.hh	100.00% <ø> (ø)
src/celeritas/ext/GeantSetup.cc	92.72% <100.00%> (+0.27%)	:arrow_up:
src/celeritas/io/ImportData.hh	100.00% <ø> (ø)
src/celeritas/inp/PhysicsProcess.hh	0.00% <0.00%> (ø)
src/celeritas/ext/GeantImporter.cc	92.82% <16.66%> (-0.59%)	:arrow_down:
src/celeritas/g4/SupportedEmStandardPhysics.cc	76.92% <59.09%> (-2.50%)	:arrow_down:
src/celeritas/ext/GeantPhysicsOptions.hh	85.29% <66.66%> (-0.58%)	:arrow_down:
src/celeritas/inp/Physics.hh	18.18% <0.00%> (-61.82%)	:arrow_down:

... and 4 files with indirect coverage changes

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