feat: Add support for reading unit cell dimensions from MOL2 CRYSIN records

[Dreamstick9]

Fixes #3341

Changes made in this Pull Request:

Implemented logic in MOL2Parser (package/MDAnalysis/coordinates/MOL2.py) to parse @CRYSIN records.

Extracted unit cell dimensions (lengths and angles) from the CRYSIN block and assigned them to ts.dimensions.

Added a new test data file with a CRYSIN record: testsuite/MDAnalysisTests/data/mol2/test_crysin.mol2.

Registered the new test file in testsuite/MDAnalysisTests/datafiles.py.

Added a regression test test_mol2_crysin_dimensions in testsuite/MDAnalysisTests/coordinates/test_mol2.py to verify dimensions are loaded correctly.

PR Checklist
[x] Issue raised/referenced?

[x] Tests updated/added?

[ ] Documentation updated/added?

[ ] package/CHANGELOG file updated?

[ ] Is your name in package/AUTHORS? (If it is not, add it!)

Developers Certificate of Origin
I certify that I can submit this code contribution as described in the Developer Certificate of Origin, under the MDAnalysis LICENSE.

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📚 Documentation preview 📚: https://mdanalysis--5187.org.readthedocs.build/en/5187/

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.72%. Comparing base (9fb8b0a) to head (a07de61).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #5187   +/-   ##
========================================
  Coverage    92.72%   92.72%           
========================================
  Files          180      180           
  Lines        22472    22475    +3     
  Branches      3188     3189    +1     
========================================
+ Hits         20838    20841    +3     
  Misses        1176     1176           
  Partials       458      458           

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