Add new div upper lower vars

[chris-ashe]

This pull request refactors how the number of divertors (n_divertors) is managed throughout the codebase. The main change is to move the ownership of n_divertors from physics_variables to a new home in divertor_variables, and to update all references accordingly. Additionally, new attributes for tracking upper and lower divertor heat loads are introduced, and relevant initialization and output routines are updated to use the new structure.

The most important changes include:

Refactoring and Data Structure Updates:

• Moved n_divertors from physics_variables to divertor_variables, removed its definition and initialization from physics_variables.py, and updated all code to reference divertor_variables.n_divertors instead of physics_variables.n_divertors.

• Updated the initialization logic in init.py to set divertor_variables.n_divertors instead of physics_variables.n_divertors based on the i_single_null flag. [1] [2]

Enhancements to Divertor Variables:

• Added new attributes to divertor_variables.py for tracking lower and upper divertor neutron nuclear heat loads and incident radiation power (p_div_lower_nuclear_heat_mw, p_div_upper_nuclear_heat_mw, p_div_upper_rad_mw, p_div_lower_rad_mw), and initialized them in init_divertor_variables().

• Implemented new run methods for LowerDivertor and UpperDivertor classes to calculate and store these new heat load and radiation attributes.

Imports and Consistency:

• Updated imports in relevant modules to ensure divertor_variables is available where needed, and replaced usage of physics_variables.n_divertors with divertor_variables.n_divertors throughout the codebase for consistency.

These changes centralize divertor-specific configuration and results within divertor_variables, improving code organization and maintainability.## 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 56.25000% with 21 lines in your changes missing coverage. Please review.
✅ Project coverage is 46.51%. Comparing base (5ea7543) to head (3b1cd7a).
⚠️ Report is 73 commits behind head on main.

Files with missing lines	Patch %	Lines
process/physics.py	0.00%	11 Missing ⚠️
process/divertor.py	33.33%	6 Missing ⚠️
process/build.py	0.00%	3 Missing ⚠️
process/init.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4019      +/-   ##
==========================================
+ Coverage   46.49%   46.51%   +0.01%     
==========================================
  Files         123      123              
  Lines       28777    28798      +21     
==========================================
+ Hits        13381    13396      +15     
- Misses      15396    15402       +6     

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[j-a-foster]

Happy with variable changes.