Add PFConductorModel enum and refactor related code for clarity

[chris-ashe]

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 79.41176% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 48.85%. Comparing base (aada716) to head (674ce07).

Files with missing lines	Patch %	Lines
process/models/pfcoil.py	63.63%	4 Missing ⚠️
process/models/tfcoil/base.py	33.33%	2 Missing ⚠️
process/models/power.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4372      +/-   ##
==========================================
+ Coverage   48.83%   48.85%   +0.01%     
==========================================
  Files         151      151              
  Lines       29376    29385       +9     
==========================================
+ Hits        14347    14356       +9     
  Misses      15029    15029              

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