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 59.20680% with 144 lines in your changes missing coverage. Please review.
✅ Project coverage is 48.65%. Comparing base (0d00ff1) to head (f0b9e2a).

Files with missing lines	Patch %	Lines
process/core/scan.py	5.08%	56 Missing ⚠️
process/core/solver/solver_handler.py	0.00%	18 Missing ⚠️
process/core/solver/solver.py	18.75%	13 Missing ⚠️
process/core/caller.py	0.00%	9 Missing ⚠️
process/core/final.py	0.00%	6 Missing ⚠️
process/core/solver/iteration_variables.py	66.66%	6 Missing ⚠️
process/core/solver/evaluators.py	0.00%	5 Missing ⚠️
process/core/init.py	90.24%	4 Missing ⚠️
process/core/io/vary_run/config.py	33.33%	4 Missing ⚠️
process/main.py	66.66%	4 Missing ⚠️
... and 12 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4306      +/-   ##
==========================================
- Coverage   48.79%   48.65%   -0.14%     
==========================================
  Files         151      151              
  Lines       29339    29267      -72     
==========================================
- Hits        14315    14239      -76     
- Misses      15024    15028       +4     

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