Add SOL physics class

[chris-ashe]

This pull request introduces a new dedicated model for scrape-off layer (SOL) physics in the codebase, providing consistent calculation, storage, and output of SOL power decay lengths using established scalings. It also adds documentation and new plotting functionality to visualize and compare these quantities. The most important changes are grouped below.

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Scrape-off Layer Model Integration

• Added a new ScrapeOffLayer model (process/models/physics/scrape_off_layer.py) that implements calculation methods for three SOL power decay length scalings (Eich 2013, MAST 2014 I, MAST 2014 II), updates the data structure, and provides output routines.
• Integrated the ScrapeOffLayer model into the main workflow, including initialization in main.py, inclusion in the Physics model, and invocation in the main model sequence.

Data Structure and Output

• Added new physics variables to store SOL power decay lengths in PhysicsData (len_plasma_sol_eich13_power_decay, len_plasma_sol_mast14_power_decay_1, len_plasma_sol_mast14_power_decay_2).
• Updated the divertor model to use the new SOL power decay length calculation and to instantiate the ScrapeOffLayer model.

Documentation

• Added a new documentation page describing the SOL power decay length scalings, equations, and references.
• Linked the new SOL documentation in the navigation menu.

Visualization

• Added a new plotting function to visualize and compare the three SOL power decay length scalings, and included this plot in the summary page.

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 46.87500% with 34 lines in your changes missing coverage. Please review.
✅ Project coverage is 48.63%. Comparing base (ef96cb5) to head (53b76ab).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
process/core/io/plot/summary.py	4.00%	24 Missing ⚠️
process/models/physics/scrape_off_layer.py	71.42%	8 Missing ⚠️
process/models/physics/physics.py	33.33%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4479      +/-   ##
==========================================
- Coverage   48.64%   48.63%   -0.02%     
==========================================
  Files         151      152       +1     
  Lines       29703    29769      +66     
==========================================
+ Hits        14449    14478      +29     
- Misses      15254    15291      +37     

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