Add MuCF muonic molecule selector and fusion channel selectors

[stognini]

This PR adds two components to the muon-catalyzed fusion executor, as a part of #1508 :

• MuonicMoleculeSelector: Given a muonic atom and selected spin, this helper class selects a final muonic molecule and exponentially samples the final cycle time accordingly.
• [DD/DT/TT]ChannelSelector: selects the final channel that the fusion will undergo.

The MuonicMoleculeSelector allows for an executor-like test, in which, from material data, we sample the atom formation, and from the selected atom, the molecule formation. The number of formed $(dd)_\mu$, $(dt)_\mu$, and $(tt)_\mu$, in our executor should be statistically equivalent to Acceleron's simulation in Geant4. And we do get similar numbers, despite a small bias towards $(tt)_\mu$ that will be investigated.

Code	$(dd)_\mu$ (%)	$(dt)_\mu$ (%)	$(tt)_\mu$ (%)
Geant4 (Acceleron)	20.4	78.8	0.8
Celeritas	20.4	79.4	0.2

[github-actions]

Test summary

 5 950 files   9 569 suites   19m 20s ⏱️
 2 196 tests  2 166 ✅  29 💤 1 ❌
32 914 runs  32 772 ✅ 141 💤 1 ❌

For more details on these failures, see this check.

Results for commit a541e4d.

♻️ This comment has been updated with latest results.

[sethrj]

Great!

[codecov]

Codecov Report

❌ Patch coverage is 85.14851% with 15 lines in your changes missing coverage. Please review.
✅ Project coverage is 87.23%. Comparing base (4b84967) to head (a541e4d).

Files with missing lines	Patch %	Lines
src/celeritas/mucf/executor/DTMixMucfExecutor.hh	0.00%	13 Missing and 1 partial ⚠️
...tas/mucf/executor/detail/MuonicMoleculeSelector.hh	97.72%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2275      +/-   ##
===========================================
+ Coverage    87.15%   87.23%   +0.08%     
===========================================
  Files         1353     1352       -1     
  Lines        43141    43179      +38     
  Branches     13195    13208      +13     
===========================================
+ Hits         37601    37669      +68     
+ Misses        4319     4282      -37     
- Partials      1221     1228       +7     

Files with missing lines	Coverage Δ
src/celeritas/inp/MucfPhysics.cc	100.00% <ø> (ø)
src/celeritas/mucf/data/DTMixMucfData.hh	100.00% <100.00%> (ø)
...eleritas/mucf/executor/detail/DDChannelSelector.hh	100.00% <100.00%> (+100.00%)	⬆️
...eleritas/mucf/executor/detail/DTChannelSelector.hh	100.00% <100.00%> (+100.00%)	⬆️
...eleritas/mucf/executor/detail/TTChannelSelector.hh	100.00% <100.00%> (+100.00%)	⬆️
src/celeritas/mucf/interactor/DTMucfInteractor.hh	91.11% <ø> (-2.23%)	⬇️
...eleritas/mucf/model/detail/MucfMaterialInserter.cc	94.44% <100.00%> (-0.20%)	⬇️
...tas/mucf/executor/detail/MuonicMoleculeSelector.hh	97.72% <97.72%> (ø)
src/celeritas/mucf/executor/DTMixMucfExecutor.hh	0.00% <0.00%> (ø)

... and 10 files with indirect coverage changes

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[sethrj]

@stognini Looks like the only remaining task is to disable testing the values when using single precision.