Clean parsers

[brynpickering]

Refactoring of the expression and where string parsers to be easier to maintain in future.

Summary of changes in this pull request:

• Move to an abstract base class for our parsers, so they have the ability to output an xarray dataarray or a string (for latex math), and anything downstream can expect one of those types
• Move constraint expression setting to the expression parsing, so that we get one dataarray (of comparison expressions) instead of an lhs/rhs. Keeps the data structure in line with the other optimisation problem components.
• Rename as_latex to as_math_string which I think is more useful a description.

Reviewer checklist:

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

[sjpfenninger]

Maybe also rename the TestAsLatex class

The base branch was changed.

[codecov]

Codecov Report

Merging #506 (dfc74b6) into main (45436c9) will increase coverage by 0.29%.
The diff coverage is 98.02%.

@@            Coverage Diff             @@
##             main     #506      +/-   ##
==========================================
+ Coverage   92.42%   92.72%   +0.29%     
==========================================
  Files          32       32              
  Lines        4120     4204      +84     
  Branches      969      965       -4     
==========================================
+ Hits         3808     3898      +90     
+ Misses        197      194       -3     
+ Partials      115      112       -3     

Files	Coverage Δ
src/calliope/backend/backend_model.py	97.46% <100.00%> (+0.05%)	⬆️
src/calliope/backend/latex_backend_model.py	96.00% <100.00%> (-0.10%)	⬇️
src/calliope/backend/parsing.py	98.66% <100.00%> (+0.04%)	⬆️
src/calliope/backend/pyomo_backend_model.py	95.67% <100.00%> (+0.01%)	⬆️
src/calliope/core/model.py	93.71% <100.00%> (ø)
src/calliope/backend/helper_functions.py	97.81% <97.14%> (-0.39%)	⬇️
src/calliope/backend/expression_parser.py	98.00% <98.46%> (+2.56%)	⬆️
src/calliope/backend/where_parser.py	97.67% <96.61%> (-0.85%)	⬇️