tf coil var 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 72.44898% with 216 lines in your changes missing coverage. Please review.
✅ Project coverage is 48.95%. Comparing base (b97ed80) to head (85e3451).

Files with missing lines	Patch %	Lines
process/models/stellarator/coils/calculate.py	12.24%	43 Missing ⚠️
process/models/tfcoil/resistive.py	31.14%	42 Missing ⚠️
process/core/init.py	55.38%	29 Missing ⚠️
process/models/tfcoil/base.py	50.00%	27 Missing ⚠️
process/models/power.py	11.76%	15 Missing ⚠️
process/models/stellarator/coils/mass.py	31.81%	15 Missing ⚠️
process/core/solver/constraints.py	56.52%	10 Missing ⚠️
process/core/scan.py	0.00%	9 Missing ⚠️
process/core/caller.py	0.00%	3 Missing ⚠️
process/core/output.py	0.00%	3 Missing ⚠️
... and 10 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4279      +/-   ##
==========================================
- Coverage   49.35%   48.95%   -0.40%     
==========================================
  Files         151      151              
  Lines       29407    29180     -227     
==========================================
- Hits        14514    14286     -228     
- Misses      14893    14894       +1     

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