4193 revamp beam fusion calculations

[grmtrkngtn]

Description

Refactored beam_fusion model.

The model now directly computes beam alpha power from:

• fast beam ion density (from a slowing-down model)
• beam-weighted thermal target density
• beam-target fusion reactivity
• plasma volume

The previous thermal reactivity correction (Bosch-Hale) has been removed.

This was not physically consistent:

• thermal fusion involves two Maxwellian populations

• beam-target fusion involves one fast population and one thermal population

• the beam reactivity already includes fast-ion averaging

• See the docs for outline of the routine. I haven't changed much else of the physics but it's been listed more clearly in the docs in the new functional form.

• Most of the changes are clarifying what each function is actually calculating and changing docstring and name accordingly.

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

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 52.11%. Comparing base (4c99916) to head (0d1c31f).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4200      +/-   ##
==========================================
+ Coverage   52.07%   52.11%   +0.03%     
==========================================
  Files         148      148              
  Lines       30321    30382      +61     
==========================================
+ Hits        15791    15834      +43     
- Misses      14530    14548      +18     

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[je-cook]

Just comments on code and docstrings. I havent reviewed the model changes

[je-cook]

Codewise happy, leaving the model thoughts to @chris-ashe