Add cable void variation in stellarator

[jjwalkowiak]

Description

I added an iteration variable to change the fraction of helium (also called void fraction) in the cable/winding pack. The reason behind this is that many stellarator builds are limited by the max B field on coils. To lower max B, PROCESS try to increase the coil cross-section. So far it was possible by increased copper fraction, which led to extremely long quench discharge times. To decouple quench time from max B on coil, I introduced an option to add more void to the cables.

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.

[grmtrkngtn]

Looks good to me.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 48.63%. Comparing base (f95b094) to head (c5aba96).
⚠️ Report is 44 commits behind head on main.

Files with missing lines	Patch %	Lines
process/models/stellarator/build.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4455      +/-   ##
==========================================
- Coverage   48.82%   48.63%   -0.20%     
==========================================
  Files         151      151              
  Lines       29408    29706     +298     
==========================================
+ Hits        14359    14448      +89     
- Misses      15049    15258     +209     

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