Add iohcl enum

[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 61.70213% with 18 lines in your changes missing coverage. Please review.
✅ Project coverage is 50.33%. Comparing base (95c71b6) to head (a5cd18a).

Files with missing lines	Patch %	Lines
process/models/pfcoil.py	53.84%	12 Missing ⚠️
process/core/io/plot/summary/magnets/pf.py	20.00%	4 Missing ⚠️
process/core/io/plot/summary/magnets/cs.py	50.00%	1 Missing ⚠️
process/models/stellarator/initialization.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4679      +/-   ##
==========================================
+ Coverage   50.31%   50.33%   +0.01%     
==========================================
  Files         181      181              
  Lines       30458    30466       +8     
==========================================
+ Hits        15326    15334       +8     
  Misses      15132    15132              

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