Convert pf power vars to dataclass

[clmould]

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

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 50.16%. Comparing base (1648e60) to head (06bccb6).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4260      +/-   ##
==========================================
- Coverage   50.18%   50.16%   -0.02%     
==========================================
  Files         151      151              
  Lines       29373    29363      -10     
==========================================
- Hits        14740    14730      -10     
  Misses      14633    14633              

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