Convert power 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 90.34707% with 89 lines in your changes missing coverage. Please review.
✅ Project coverage is 50.24%. Comparing base (4290d86) to head (7a30f17).

Files with missing lines	Patch %	Lines
process/models/power.py	45.76%	32 Missing ⚠️
process/models/ife.py	90.56%	25 Missing ⚠️
process/models/costs/costs.py	51.28%	19 Missing ⚠️
process/core/scan.py	0.00%	4 Missing ⚠️
process/core/caller.py	25.00%	3 Missing ⚠️
process/core/init.py	75.00%	3 Missing ⚠️
process/core/output.py	0.00%	1 Missing ⚠️
process/models/stellarator/initialization.py	0.00%	1 Missing ⚠️
process/models/structure.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4257      +/-   ##
==========================================
- Coverage   50.66%   50.24%   -0.42%     
==========================================
  Files         151      151              
  Lines       29615    29367     -248     
==========================================
- Hits        15004    14756     -248     
  Misses      14611    14611              

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