Create plasma current class

[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 75.43860% with 28 lines in your changes missing coverage. Please review.
✅ Project coverage is 47.25%. Comparing base (4741c15) to head (248115d).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
process/models/physics/plasma_current.py	76.41%	25 Missing ⚠️
process/models/physics/physics.py	50.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4086      +/-   ##
==========================================
+ Coverage   46.88%   47.25%   +0.36%     
==========================================
  Files         136      139       +3     
  Lines       29253    29365     +112     
==========================================
+ Hits        13715    13875     +160     
+ Misses      15538    15490      -48     

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