Add Gauss/Lorentz model switch to Becker-Coppens extinction

[AndrewSazonov]

The extinction correction was incorrectly labelled as Shelx-style. CrysPy implements analytical Becker-Coppens spherical extinction (primary x secondary), not the empirical Shelx model.

• Rename ShelxExtinction -> BeckerCoppensExtinction (tag becker-coppens)
• Add model StringDescriptor to switch between 'gauss' and 'lorentz' mosaicity distributions

[codecov]

Codecov Report

❌ Patch coverage is 80.00000% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.17%. Comparing base (58b4b29) to head (071894f).
⚠️ Report is 1 commits behind head on develop.

Files with missing lines	Patch %	Lines
...asydiffraction/datablocks/experiment/item/enums.py	58.33%	5 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #149      +/-   ##
===========================================
- Coverage    89.21%   89.17%   -0.04%     
===========================================
  Files          134      134              
  Lines         7694     7716      +22     
  Branches       723      725       +2     
===========================================
+ Hits          6864     6881      +17     
- Misses         600      605       +5     
  Partials       230      230              

Flag	Coverage Δ
integration	74.96% <76.00%> (-0.01%)	⬇️
unittests	79.89% <76.00%> (-0.01%)	⬇️

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