Create fusion reaction file

[chris-ashe]

This pull request includes changes to the process/physics.py file and its corresponding unit tests to refactor the import and usage of fusion reaction functions. The most important changes involve moving fusion reaction functions from process.physics_functions to a new module process.fusion_reactions.

Refactor fusion reaction functions:

• process/physics.py: Changed import from process.physics_functions to process.fusion_reactions and updated function calls accordingly.

Unit test updates:

• tests/unit/test_fusion_reactions.py: Renamed from tests/unit/test_physics_functions.py and updated imports and function calls to use process.fusion_reactions instead of process.physics_functions.

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

Attention: Patch coverage is 59.09091% with 90 lines in your changes missing coverage. Please review.

Project coverage is 31.74%. Comparing base (cf82f05) to head (d9c259c).
Report is 136 commits behind head on main.

Files with missing lines	Patch %	Lines
process/fusion_reactions.py	59.06%	88 Missing ⚠️
process/physics.py	50.00%	1 Missing ⚠️
process/stellarator.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3599      +/-   ##
==========================================
+ Coverage   31.71%   31.74%   +0.02%     
==========================================
  Files          84       85       +1     
  Lines       20152    20160       +8     
==========================================
+ Hits         6392     6400       +8     
  Misses      13760    13760              

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[timothy-nunn]

No changes to output just organises the code into more sensibly named files