Convert numerics 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

❌ Patch coverage is 55.93220% with 104 lines in your changes missing coverage. Please review.
✅ Project coverage is 48.79%. Comparing base (5bc1887) to head (82c02e8).

Files with missing lines	Patch %	Lines
process/core/scan.py	0.00%	33 Missing ⚠️
process/core/solver/solver_handler.py	0.00%	17 Missing ⚠️
process/core/solver/solver.py	8.33%	11 Missing ⚠️
process/core/final.py	0.00%	6 Missing ⚠️
process/core/solver/iteration_variables.py	66.66%	6 Missing ⚠️
process/core/caller.py	0.00%	4 Missing ⚠️
process/core/io/vary_run/config.py	0.00%	4 Missing ⚠️
process/core/solver/evaluators.py	0.00%	4 Missing ⚠️
process/models/physics/physics.py	0.00%	4 Missing ⚠️
process/core/io/vary_run/tools.py	40.00%	3 Missing ⚠️
... and 9 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4306      +/-   ##
==========================================
- Coverage   48.89%   48.79%   -0.11%     
==========================================
  Files         151      151              
  Lines       29439    29383      -56     
==========================================
- Hits        14395    14337      -58     
- Misses      15044    15046       +2     

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