Create confinement time class

[chris-ashe]

This pull request refactors how confinement time models are accessed and labeled throughout the codebase. The main improvement is the centralization of confinement time calculations and model labels into the PlasmaConfinementTime and ConfinementTimeModel classes, which increases code maintainability and clarity. Additionally, hardcoded label lists have been removed, and references to confinement time models are now made through well-defined class properties.

Key changes:

Refactoring confinement time model access:

• All calls to confinement time scaling functions in plot_proc.py are now accessed via the PlasmaConfinementTime class, replacing the previous use of the confine module. This change standardizes how these models are instantiated and used.

Centralizing and improving model labeling:

• The hardcoded LABELS_CONFINEMENT_SCALINGS list has been removed from physics_variables.py, and references to confinement time model labels in plot_proc.py are now made through the ConfinementTimeModel class's full_name property. This ensures label consistency and reduces duplication.

Initialization and dependency injection improvements:

• The PlasmaConfinementTime class is now instantiated and injected as a dependency into the Physics class in main.py, making it easier to manage and test.

API updates for downstream usage:

• The calculate_confinement_time method in the stellarator model is now accessed via self.physics.confinement, reflecting the new structure for confinement time calculations.

Documentation and cleanup:

• The docstring for i_confinement_time in physics_variables.py has been updated to remove references to the deleted label list, reflecting the new approach to model labeling.## 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 30.00000% with 21 lines in your changes missing coverage. Please review.
✅ Project coverage is 47.73%. Comparing base (67340b1) to head (07e7a7b).

Files with missing lines	Patch %	Lines
process/core/io/plot_proc.py	9.52%	19 Missing ⚠️
process/models/physics/physics.py	66.66%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4121      +/-   ##
==========================================
+ Coverage   47.51%   47.73%   +0.21%     
==========================================
  Files         141      141              
  Lines       29487    29628     +141     
==========================================
+ Hits        14010    14142     +132     
- Misses      15477    15486       +9     

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