Add ITER normalised Inductance l_i(3)

[chris-ashe]

This pull request adds support for the ITER Version 3 $l_i(3)$ definition of plasma normalised internal inductance throughout the codebase and documentation. It introduces a new calculation method, exposes the result as a new variable, updates plotting and output routines, and provides documentation and testing for this new feature.

ITER li(3) inductance support:

• Added the calculate_normalised_internal_inductance_iter_3 static method to the Physics class, implementing the ITER Version 3 definition for normalised internal inductance.
• Integrated the new calculation into the main physics workflow, storing the result in the new variable ind_plasma_internal_norm_iter_3.
• Declared, initialized, and documented the new variable ind_plasma_internal_norm_iter_3 in physics_variables.py, and included it in the initialization routine.

Output and visualization:

• Updated the plasma information plotting function to display the new ITER li(3) value alongside existing inductance values.
• Modified the output routine to write the ITER li(3) value to output files.

Documentation and testing:

• Added a section to the documentation describing the ITER li(3) definition, its formula, and relevant references.
• Added a unit test to verify the correctness of the new calculation method.
• Promoted the Menard inductance relation heading for clarity in the documentation.

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 75.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 43.17%. Comparing base (8dd9e07) to head (f2a3024).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
process/physics.py	60.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3974      +/-   ##
==========================================
- Coverage   46.05%   43.17%   -2.89%     
==========================================
  Files         123      123              
  Lines       28738    31217    +2479     
==========================================
+ Hits        13235    13477     +242     
- Misses      15503    17740    +2237     

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[j-a-foster]

Happy with changes.