Convert scan variables 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 65.11628% with 45 lines in your changes missing coverage. Please review.
✅ Project coverage is 48.73%. Comparing base (0b7e14f) to head (1285468).

Files with missing lines	Patch %	Lines
process/core/scan.py	14.81%	23 Missing ⚠️
process/core/caller.py	0.00%	5 Missing ⚠️
process/core/solver/solver.py	44.44%	5 Missing ⚠️
process/core/init.py	66.66%	3 Missing ⚠️
process/core/io/vary_run/config.py	50.00%	2 Missing ⚠️
process/main.py	75.00%	2 Missing ⚠️
process/core/process_output.py	80.00%	1 Missing ⚠️
process/core/solver/evaluators.py	0.00%	1 Missing ⚠️
process/core/solver/solver_handler.py	0.00%	1 Missing ⚠️
process/models/costs/costs_2015.py	0.00%	1 Missing ⚠️
... and 1 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4305      +/-   ##
==========================================
- Coverage   48.76%   48.73%   -0.04%     
==========================================
  Files         151      151              
  Lines       29328    29314      -14     
==========================================
- Hits        14301    14285      -16     
- Misses      15027    15029       +2     

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