feat: support nanoparticle pdf refinement

[ycexiao]

What problem does this PR address?

Add diffpy.apps interface to solve the example case diffpy.srfit/docs/example/nppdfcrystal.py

What should the reviewer(s) do?

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.03%. Comparing base (76ca2b3) to head (62630c0).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #33      +/-   ##
==========================================
+ Coverage   97.43%   98.03%   +0.59%     
==========================================
  Files           8        8              
  Lines         429      559     +130     
==========================================
+ Hits          418      548     +130     
  Misses         11       11              

Files with missing lines	Coverage Δ
tests/helper.py	98.94% <100.00%> (+0.19%)	⬆️
tests/test_parametric_model.py	100.00% <100.00%> (ø)
tests/test_refinement_server.py	100.00% <100.00%> (ø)
tests/test_refinement_session.py	100.00% <100.00%> (ø)

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[ycexiao]

@sbillinge it's ready for review.

[ycexiao]

@sbillinge, it's ready to merge.