Cleanup names and math docs

[brynpickering]

Minor renaming to be a bit clearer

Summary of changes in this pull request:

• objective_cost_class -> objective_cost_weights
• minmax_cost_optimisation -> min_cost_optimisation (since it is minimising in the base math)
• fixed math doc generation. Will need to fold this into the docs so that we don't keep missing failing parts of this...

Reviewer checklist:

• Test(s) added to cover contribution
• Documentation updated
• Changelog updated
• Coverage maintained or improved

[codecov]

Codecov Report

Merging #513 (8d6998f) into main (3779e0d) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #513   +/-   ##
=======================================
  Coverage   93.21%   93.21%           
=======================================
  Files          28       28           
  Lines        3817     3817           
  Branches      877      877           
=======================================
  Hits         3558     3558           
  Misses        168      168           
  Partials       91       91           

Files	Coverage Δ
src/calliope/preprocess/checks.py	85.16% <100.00%> (ø)
src/calliope/preprocess/model_run.py	91.82% <ø> (ø)