🧲 Added plasma magnetic energy

[chris-ashe]

This pull request introduces the calculation and tracking of plasma stored magnetic energy throughout the codebase. The main changes include adding a new variable, computing its value, and updating outputs and plots to include this information.

Addition and handling of plasma stored magnetic energy:

• Added new variable e_plasma_magnetic_stored to physics_variables.py to represent plasma stored magnetic energy, including its initialization in init_physics_variables() and setting its default value to 0.0.
• Computed e_plasma_magnetic_stored in the physics method as 0.5 * ind_plasma * plasma_current ** 2.

Output and visualization updates:

• Included plasma stored magnetic energy in the output file via the outplas method, using the ovarre function to write its value.
• Updated the main plasma information plot to display plasma stored magnetic energy (converted to GJ) alongside other plasma parameters.## 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 60.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 46.40%. Comparing base (9fc4b8b) to head (5aef984).

Files with missing lines	Patch %	Lines
process/physics.py	0.00%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3906   +/-   ##
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  Coverage   46.40%   46.40%           
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  Files         123      123           
  Lines       28173    28178    +5     
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+ Hits        13073    13076    +3     
- Misses      15100    15102    +2     

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