misc functionalities

[ycexiao]

What problem does this PR address?

• feat: allow add structure from code string
• feat: support soft bounds and generator run in parallel
• feat: support fetching cif files

What should the reviewer(s) do?

[ycexiao]

Closed as it is the wrong branch to submit.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.24%. Comparing base (b82903c) to head (4b6d413).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #35      +/-   ##
==========================================
+ Coverage   98.21%   98.24%   +0.03%     
==========================================
  Files           8        8              
  Lines         615      626      +11     
==========================================
+ Hits          604      615      +11     
  Misses         11       11              

Files with missing lines	Coverage Δ
tests/test_refinement_session.py	100.00% <100.00%> (ø)

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