Update cost expression names & split out expressions

[brynpickering]

Fixes #645

The only thing I have chosen to not implement from #645 is cost_investment -> cost_investment_total. I'm not sure that _total adds anything...

Summary of changes in this pull request

• the total capital cost is now cost_investment
• the annualised capital cost is now cost_investment_annualised
• fixed operation costs are now cost_operation_fixed
• variable operation costs are now cost_operation_variable
• total cost is the sum of cost_investment_annualised, cost_operation_fixed, and cost_operation_variable.

Reviewer checklist

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

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.95%. Comparing base (c00486b) to head (fd20445).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #685   +/-   ##
=======================================
  Coverage   95.95%   95.95%           
=======================================
  Files          29       29           
  Lines        4025     4026    +1     
  Branches      845      775   -70     
=======================================
+ Hits         3862     3863    +1     
  Misses         72       72           
  Partials       91       91           

Files with missing lines	Coverage Δ
src/calliope/backend/helper_functions.py	96.86% <100.00%> (+0.01%)	⬆️

[brynpickering]

@sstroemer can you check that this is what you were thinking w.r.t. updating expressions?

[sstroemer]

@sstroemer can you check that this is what you were thinking w.r.t. updating expressions?

@brynpickering Looks really good!

[sjpfenninger]

Looks good to me, much clearer than before!