Add PlasmaDensityLimit class and integrate into physics and stellarat…

[chris-ashe]

…or models

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 60.82474% with 38 lines in your changes missing coverage. Please review.
✅ Project coverage is 46.58%. Comparing base (774b255) to head (d46b934).

Files with missing lines	Patch %	Lines
process/physics.py	61.95%	35 Missing ⚠️
process/io/plot_proc.py	0.00%	2 Missing ⚠️
process/main.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4093      +/-   ##
==========================================
+ Coverage   46.56%   46.58%   +0.01%     
==========================================
  Files         124      124              
  Lines       28980    29061      +81     
==========================================
+ Hits        13495    13538      +43     
- Misses      15485    15523      +38     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.