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

❌ Patch coverage is 71.42857% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.72%. Comparing base (4c12f15) to head (a4c6bb1).

Files with missing lines	Patch %	Lines
package/MDAnalysis/coordinates/MOL2.py	71.42%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #5187      +/-   ##
===========================================
- Coverage    92.72%   92.72%   -0.01%     
===========================================
  Files          180      180              
  Lines        22475    22482       +7     
  Branches      3190     3191       +1     
===========================================
+ Hits         20841    20846       +5     
- Misses        1177     1179       +2     
  Partials       457      457              

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[orbeckst]

@Dreamstick9 can you link to the document that describes the @CRYSIN record? Can you show/link to examples mol2 files found "in the wild" that use it, e.g., from a database?

That may help with reviewing.

Also make sure that you address all points in the checklist (eg, add a note to the docs, add an entry to CHANGELOG). Reviewing PRs takes a lot of time and time is really precious so show reviewers that you've done all the necessary work.

[Dreamstick9]

I implemented this based on the official Tripos mol2 file format specification, the @crysin record is defined the the standard manual in page 17 of the pdf(https://github.com/UnixJunkie/mol2-file-format-spec/blob/master/mol2.pdf). the test case added in mol2_crysin was for this specification. this record is the standard method for defining periodic boundary conditions in the format (see "Data Record Format" in the spec).
i have updated the CHANGELOG and AUTHORS.

Thanks for the review @orbeckst