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 85.66667% with 43 lines in your changes missing coverage. Please review.
✅ Project coverage is 47.98%. Comparing base (f2d9a54) to head (e358e2e).
⚠️ Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
process/models/costs/costs_2015.py	84.75%	43 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4153      +/-   ##
==========================================
- Coverage   48.01%   47.98%   -0.03%     
==========================================
  Files         143      143              
  Lines       30077    30147      +70     
==========================================
+ Hits        14440    14466      +26     
- Misses      15637    15681      +44     

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