Feature helper function classes

[brynpickering]

Fixes issue #419

Summary of changes in this pull request:

• Helper functions changed to classes, for 3 reasons:
  1. The actual functionality renders in the docs properly (previous implementation of private sub-function wouldn't allow the main helper function method to be rendered in the API docs)
  2. It is more robust to future additions (e.g., returning an array or a string depending on where or not latex strings are desired - #442)
  3. It is possible to add new helper functions on-the-fly. Just creating a helper function that inherits from the defined abstract base class will add it to the helper function registry, making it available for when you run calliope.Model.build. The setup of the registry means that in-built helper functions can't be easily overridden.
• Docstrings added, with some more complex ones having an example included.
• Functions rendered in API documentation.
• Some helper functions and their arguments renamed to be more descriptive - @jnnr would be good to know if you agree that they are actually more descriptive!

Reviewer checklist:

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

[codecov[bot]]

Codecov Report

Merging #467 (583f761) into main (7e4e973) will increase coverage by 0.10%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #467      +/-   ##
==========================================
+ Coverage   94.91%   95.01%   +0.10%     
==========================================
  Files          31       31              
  Lines        3834     3875      +41     
==========================================
+ Hits         3639     3682      +43     
+ Misses        195      193       -2     

Impacted Files	Coverage Δ
calliope/backend/parsing.py	100.00% <ø> (ø)
calliope/core/model.py	95.83% <ø> (ø)
calliope/backend/expression_parser.py	97.80% <100.00%> (+0.01%)	:arrow_up:
calliope/backend/helper_functions.py	100.00% <100.00%> (ø)
calliope/backend/where_parser.py	100.00% <100.00%> (+1.01%)	:arrow_up: