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 95.98%. Comparing base (62dcd34) to head (19a63f8).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #685   +/-   ##
=======================================
  Coverage   95.98%   95.98%           
=======================================
  Files          26       26           
  Lines        3981     3982    +1     
  Branches      837      768   -69     
=======================================
+ Hits         3821     3822    +1     
  Misses         70       70           
  Partials       90       90           

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