Refactor fusion reactions for dataclass

[chris-ashe]

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 39.64758% with 137 lines in your changes missing coverage. Please review.
✅ Project coverage is 50.12%. Comparing base (4618fd4) to head (4b99c28).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
process/models/physics/fusion_reactions.py	34.00%	99 Missing ⚠️
process/core/io/plot/summary.py	0.00%	22 Missing ⚠️
process/models/physics/physics.py	20.00%	12 Missing ⚠️
process/models/stellarator/stellarator.py	0.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4592      +/-   ##
==========================================
+ Coverage   49.91%   50.12%   +0.21%     
==========================================
  Files         151      151              
  Lines       29860    29937      +77     
==========================================
+ Hits        14904    15007     +103     
+ Misses      14956    14930      -26     

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