Perform power_at_ignition_point on a proxy stellarator data structure

[timothy-nunn]

Removes the need for the stellarator power at ignition model to modify the data structure, run physics, extract results, reset the data structure, and then run physics to converge back to the old point. Instead, we make a copy of the data structure, mutate this instance of the data, run the models, extract the data of interest, and then discard the mutated structure.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 11.11111% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 48.65%. Comparing base (260e0e1) to head (aa81b27).

Files with missing lines	Patch %	Lines
process/models/stellarator/denisty_limits.py	12.50%	7 Missing ⚠️
process/models/stellarator/stellarator.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4291      +/-   ##
==========================================
+ Coverage   48.64%   48.65%   +0.01%     
==========================================
  Files         151      151              
  Lines       29283    29274       -9     
==========================================
  Hits        14244    14244              
+ Misses      15039    15030       -9     

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