move data_structure_dicts, obsolete_variables and variable_metadata t…

[clmould]

…o process/core/data_structure

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 49.85%. Comparing base (a6cc0e2) to head (74e6bf7).
⚠️ Report is 7 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4562      +/-   ##
==========================================
- Coverage   49.87%   49.85%   -0.03%     
==========================================
  Files         151      151              
  Lines       29873    29875       +2     
==========================================
- Hits        14899    14894       -5     
- Misses      14974    14981       +7     

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[timothy-nunn]

This file could be renamed now, the data_structure part is redundant