Add MuCF interactor for muonic dd molecules

[stognini]

This PR implements the muon-catalyzed fusion interactor for $(dd)_\mu$ molecules.

To avoid code duplication, a few utils helper functions are created and shared between both $(dd)_\mu$ and $(dt)_\mu$ interactors and are expected to be used by the last interactor. This is a component of #1508 .

[codecov]

Codecov Report

❌ Patch coverage is 98.30508% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.86%. Comparing base (43e05ae) to head (912af6a).
⚠️ Report is 3 commits behind head on develop.

Files with missing lines	Patch %	Lines
src/celeritas/mucf/interactor/DDMucfInteractor.hh	96.36%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2236      +/-   ##
===========================================
+ Coverage    86.81%   86.86%   +0.05%     
===========================================
  Files         1330     1331       +1     
  Lines        42019    42102      +83     
  Branches     12950    12951       +1     
===========================================
+ Hits         36480    36574      +94     
+ Misses        4335     4319      -16     
- Partials      1204     1209       +5     

Files with missing lines	Coverage Δ
src/celeritas/mucf/data/DTMixMucfData.hh	91.66% <ø> (ø)
src/celeritas/mucf/interactor/DTMucfInteractor.hh	93.33% <100.00%> (-1.98%)	⬇️
...itas/mucf/interactor/detail/MucfInteractorUtils.hh	100.00% <100.00%> (ø)
src/celeritas/phys/PDGNumber.hh	96.66% <100.00%> (+7.38%)	⬆️
src/celeritas/mucf/interactor/DDMucfInteractor.hh	95.23% <96.36%> (+95.23%)	⬆️

... and 10 files with indirect coverage changes

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[github-actions]

Test summary

 5 913 files   9 451 suites   18m 37s ⏱️
 2 151 tests  2 123 ✅  28 💤 0 ❌
32 338 runs  32 180 ✅ 158 💤 0 ❌

Results for commit 912af6a.

♻️ This comment has been updated with latest results.

[sethrj]

@stognini build errors indicate you're missing some inline keywords:

multiple definition of `celeritas::detail::calc_third_secondary(celeritas::Secondary, celeritas::Quantity<celeritas::units::MevPerCsq, double>, celeritas::Secondary, celeritas::Quantity<celeritas::units::MevPerCsq, double>, celeritas::OpaqueId<celeritas::Particle_, unsigned int>, celeritas::Quantity<celeritas::units::MevPerCsq, double>)'; src/celeritas/CMakeFiles/celeritas.dir/mucf/model/DTMixMucfModel.cc.o:/__w/celeritas/celeritas/src/celeritas/mucf/interactor/detail/MucfInteractorUtils.hh:114: first defined here