move costs 2015 data structure 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 85.33333% with 44 lines in your changes missing coverage. Please review.
✅ Project coverage is 49.44%. Comparing base (893a36c) to head (859b4e3).
⚠️ Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
process/models/costs/costs_2015.py	84.45%	44 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4153      +/-   ##
==========================================
+ Coverage   48.01%   49.44%   +1.43%     
==========================================
  Files         143      149       +6     
  Lines       30164    29791     -373     
==========================================
+ Hits        14482    14729     +247     
+ Misses      15682    15062     -620     

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