Add Solov'ev equilibria

[chris-ashe]

This pull request expands the plotting functionality in process/io/plot_proc.py by adding a new analytic equilibrium plot and updating the figure indexing throughout the main plotting routine to accommodate the additional plot. The changes ensure all plots are correctly assigned to their respective figure pages and axes.

New analytic equilibrium plot:

• Added import for plot_analytic_equilibrium from process.models.physics.solovev_equilibrium and integrated its plotting into the main plotting routine, providing visualization of the analytic equilibrium. [1] [2]

Plot indexing and figure allocation updates:

• Incremented the figure count from 30 to 31 in the main function to accommodate the new plot.
• Updated subplot and figure indices throughout the main_plot function to ensure each plot is placed on the correct page and axis, maintaining layout consistency after the addition of the new plot.

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 14.13534% with 571 lines in your changes missing coverage. Please review.
✅ Project coverage is 46.19%. Comparing base (4741c15) to head (7435ed6).

Files with missing lines	Patch %	Lines
process/models/physics/solovev_equilibrium.py	14.57%	545 Missing ⚠️
process/io/plot_proc.py	3.70%	26 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4114      +/-   ##
==========================================
- Coverage   46.88%   46.19%   -0.70%     
==========================================
  Files         136      137       +1     
  Lines       29253    29895     +642     
==========================================
+ Hits        13715    13809      +94     
- Misses      15538    16086     +548     

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[je-cook]

I think a lot of this can be done far more concisely. Being this long makes is much harder to follow. My estimate is <1000 lines.

Please use dataclasses where appropriate and include less in the overall class.
I would also guess that most of the methods are not individually useful to a user. Please have a think about what is a useful API to expose to the user in the classes and only make methods for that

[je-cook]

just use contourpy directly, then we dont need the new dependency as it already comes with mpl