Reorganize surface helper functions

[sethrj]

Split from #2303

I've inlined two of the helper functions for getting material properties, as well as adding stream operators to allow easier printing in case of warnings/failures.

[codecov]

Codecov Report

❌ Patch coverage is 65.75342% with 25 lines in your changes missing coverage. Please review.
✅ Project coverage is 87.21%. Comparing base (37e853d) to head (1ba845e).
⚠️ Report is 1 commits behind head on develop.

Files with missing lines	Patch %	Lines
...eleritas/ext/detail/GeantMaterialPropertyGetter.hh	71.42%	8 Missing and 2 partials ⚠️
.../celeritas/ext/detail/GeantSurfacePhysicsHelper.hh	69.56%	6 Missing and 1 partial ⚠️
src/celeritas/ext/detail/GeantPhysicsLoader.cc	60.00%	3 Missing and 1 partial ⚠️
.../celeritas/ext/detail/GeantSurfacePhysicsLoader.cc	0.00%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2305      +/-   ##
===========================================
+ Coverage    82.23%   87.21%   +4.98%     
===========================================
  Files         1264     1357      +93     
  Lines        27355    43253   +15898     
  Branches     10507    13197    +2690     
===========================================
+ Hits         22495    37725   +15230     
- Misses        2983     4319    +1336     
+ Partials      1877     1209     -668     

Files with missing lines	Coverage Δ
src/celeritas/ext/GeantImporter.cc	91.20% <100.00%> (+3.40%)	⬆️
src/celeritas/ext/detail/GeantPhysicsLoader.cc	92.34% <60.00%> (+5.48%)	⬆️
.../celeritas/ext/detail/GeantSurfacePhysicsLoader.cc	73.01% <0.00%> (-0.28%)	⬇️
.../celeritas/ext/detail/GeantSurfacePhysicsHelper.hh	66.66% <69.56%> (ø)
...eleritas/ext/detail/GeantMaterialPropertyGetter.hh	72.22% <71.42%> (-16.02%)	⬇️

... and 1203 files with indirect coverage changes

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[github-actions]

Test summary

 5 930 files   9 549 suites   21m 42s ⏱️
 2 204 tests  2 169 ✅  35 💤 0 ❌
32 830 runs  32 689 ✅ 141 💤 0 ❌

Results for commit 1ba845e.

♻️ This comment has been updated with latest results.