Convert neoclassics vars to dataclass

[clmould]

Description

Checklist

I confirm that I have completed the following checks:

• My changes follow the PROCESS style guide
• I have justified any large differences in the regression tests caused by this pull request in the comments.
• I have added new tests where appropriate for the changes I have made.
• If I have had to change any existing unit or integration tests, I have justified this change in the pull request comments.
• If I have made documentation changes, I have checked they render correctly.
• I have added documentation for my change, if appropriate.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 84.52381% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 50.13%. Comparing base (adf43c5) to head (cc93f87).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
process/models/stellarator/neoclassics.py	60.60%	13 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4263      +/-   ##
==========================================
- Coverage   50.16%   50.13%   -0.04%     
==========================================
  Files         151      151              
  Lines       29363    29343      -20     
==========================================
- Hits        14730    14710      -20     
  Misses      14633    14633              

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