Convert superconducting tf variables 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 67.31392% with 101 lines in your changes missing coverage. Please review.
✅ Project coverage is 48.81%. Comparing base (77f8d67) to head (1b366bc).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
process/models/tfcoil/superconducting.py	9.37%	87 Missing ⚠️
process/core/caller.py	12.50%	7 Missing ⚠️
process/models/tfcoil/base.py	16.66%	5 Missing ⚠️
process/models/stellarator/coils/quench.py	0.00%	1 Missing ⚠️
process/models/tfcoil/resistive.py	94.11%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4281      +/-   ##
==========================================
- Coverage   48.95%   48.81%   -0.15%     
==========================================
  Files         151      151              
  Lines       29180    29097      -83     
==========================================
- Hits        14286    14203      -83     
  Misses      14894    14894              

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