Feature: piecewise constraints

[brynpickering]

Fixes #107

Summary of changes in this pull request:

• Add piecewise_constraints component to YAML math.
• Add simple example of its use in the user-defined math gallery.

TODO

• Handle piecewise constraints when using verbose YAML - I suspect it won't be possible to incorporate it properly.
• Handle piecewise constraints in math docs
• Capture piecewise validation errors and pipe them through our own exceptions classes.
  - [ ] Test values as lists and not as mutable parameters. EDIT: not adding
• Add documentation.

Reviewer checklist:

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

[codecov]

Codecov Report

Attention: Patch coverage is 95.41985% with 6 lines in your changes are missing coverage. Please review.

Project coverage is 96.12%. Comparing base (872978d) to head (52043a9).
Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #569      +/-   ##
==========================================
+ Coverage   95.85%   96.12%   +0.26%     
==========================================
  Files          24       24              
  Lines        3619     3689      +70     
  Branches      788      717      -71     
==========================================
+ Hits         3469     3546      +77     
+ Misses         86       84       -2     
+ Partials       64       59       -5     

Files	Coverage Δ
src/calliope/backend/backend_model.py	100.00% <100.00%> (+2.09%)	⬆️
src/calliope/backend/latex_backend_model.py	98.11% <100.00%> (+0.31%)	⬆️
src/calliope/backend/parsing.py	98.69% <100.00%> (+0.92%)	⬆️
src/calliope/backend/where_parser.py	98.17% <100.00%> (+0.87%)	⬆️
src/calliope/backend/expression_parser.py	97.36% <50.00%> (+0.18%)	⬆️
src/calliope/backend/pyomo_backend_model.py	94.82% <92.15%> (-0.60%)	⬇️

... and 3 files with indirect coverage changes