🎨 Plot sauter bootstrap profile

[chris-ashe]

This pull request adds support for storing, processing, and visualizing the Sauter bootstrap current density profile in the plasma physics workflow. The changes introduce a new variable to hold the bootstrap profile, compute and store it during physics calculations, output it for further use, and update the plotting routines to display the profile for analysis.

Support for Sauter bootstrap current density profile:

• Added a new variable j_plasma_bootstrap_sauter_profile to physics_variables.py to store the Sauter bootstrap current density profile, with initialization and global declaration updates.
• In physics.py, updated the bootstrap_fraction_sauter function to compute and assign the Sauter bootstrap current density profile to j_plasma_bootstrap_sauter_profile using numpy gradients for improved numerical stability.
• Output the computed Sauter bootstrap current density profile at each profile point to the output file for downstream use.

Visualization improvements:

• Updated the plot_jprofile function in plot_proc.py to accept the new profile data, plot the Sauter bootstrap current density profile alongside the existing current density, and add a legend and annotation for clarity.
  Technical and numerical fixes:

• Replaced manual partial derivative calculations with numpy.gradient for more robust and accurate computation of profile gradients in the Sauter bootstrap calculation.

• Minor refactoring to remove unused or redundant variables and checks in the bootstrap calculation for code clarity.

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 50.00000% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 45.87%. Comparing base (aa129eb) to head (fe94e80).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
process/io/plot_proc.py	14.28%	6 Missing ⚠️
process/physics.py	60.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3976      +/-   ##
==========================================
- Coverage   46.04%   45.87%   -0.18%     
==========================================
  Files         123      123              
  Lines       28962    30081    +1119     
==========================================
+ Hits        13336    13800     +464     
- Misses      15626    16281     +655     

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[j-a-foster]

Happy with changes to plot proc plot.