feat: support molecule PDF refinement

[ycexiao]

What problem does this PR address?

Support molecule PDF refinement:

1. Allow user upload the code to construct the pyobjcryst structure
2. Add special parameters applied to molecule.

What should the reviewer(s) do?

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.21%. Comparing base (62630c0) to head (b82903c).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #34      +/-   ##
==========================================
+ Coverage   98.03%   98.21%   +0.17%     
==========================================
  Files           8        8              
  Lines         559      615      +56     
==========================================
+ Hits          548      604      +56     
  Misses         11       11              

Files with missing lines	Coverage Δ
tests/helper.py	99.10% <100.00%> (+0.16%)	⬆️
tests/test_parametric_model.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.