Convert stellarator 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 73.01587% with 68 lines in your changes missing coverage. Please review.
✅ Project coverage is 50.20%. Comparing base (226576d) to head (ce18343).

Files with missing lines	Patch %	Lines
process/models/stellarator/coils/calculate.py	23.07%	20 Missing ⚠️
process/models/stellarator/stellarator.py	0.00%	14 Missing ⚠️
process/models/physics/physics.py	0.00%	9 Missing ⚠️
process/models/stellarator/heating.py	11.11%	8 Missing ⚠️
process/models/stellarator/divertor.py	0.00%	6 Missing ⚠️
process/models/physics/plasma_geometry.py	0.00%	3 Missing ⚠️
process/models/physics/plasma_current.py	60.00%	2 Missing ⚠️
process/core/caller.py	0.00%	1 Missing ⚠️
process/core/output.py	0.00%	1 Missing ⚠️
process/core/solver/constraints.py	50.00%	1 Missing ⚠️
... and 3 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4259      +/-   ##
==========================================
- Coverage   50.23%   50.20%   -0.04%     
==========================================
  Files         151      151              
  Lines       29365    29345      -20     
==========================================
- Hits        14752    14732      -20     
  Misses      14613    14613              

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