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

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

Reviewer checklist:

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

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 98.85057% with 2 lines in your changes missing coverage. Please review.

Project coverage is 95.97%. Comparing base (61e1afa) to head (ff81669).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #569 +/- ## ========================================== + Coverage 95.92% 95.97% +0.05% ========================================== Files 26 26 Lines 3899 3980 +81 Branches 838 767 -71 ========================================== + Hits 3740 3820 +80 - Misses 69 70 +1 Partials 90 90 ``` | [Files](https://app.codecov.io/gh/calliope-project/calliope/pull/569?dropdown=coverage&src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=calliope-project) | Coverage Δ | | |---|---|---| | [src/calliope/backend/backend\_model.py](https://app.codecov.io/gh/calliope-project/calliope/pull/569?src=pr&el=tree&filepath=src%2Fcalliope%2Fbackend%2Fbackend_model.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=calliope-project#diff-c3JjL2NhbGxpb3BlL2JhY2tlbmQvYmFja2VuZF9tb2RlbC5weQ==) | `97.95% <100.00%> (+0.29%)` | :arrow_up: | | [src/calliope/backend/gurobi\_backend\_model.py](https://app.codecov.io/gh/calliope-project/calliope/pull/569?src=pr&el=tree&filepath=src%2Fcalliope%2Fbackend%2Fgurobi_backend_model.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=calliope-project#diff-c3JjL2NhbGxpb3BlL2JhY2tlbmQvZ3Vyb2JpX2JhY2tlbmRfbW9kZWwucHk=) | `95.66% <100.00%> (+0.06%)` | :arrow_up: | | [src/calliope/backend/latex\_backend\_model.py](https://app.codecov.io/gh/calliope-project/calliope/pull/569?src=pr&el=tree&filepath=src%2Fcalliope%2Fbackend%2Flatex_backend_model.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=calliope-project#diff-c3JjL2NhbGxpb3BlL2JhY2tlbmQvbGF0ZXhfYmFja2VuZF9tb2RlbC5weQ==) | `96.83% <100.00%> (+0.56%)` | :arrow_up: | | [src/calliope/backend/parsing.py](https://app.codecov.io/gh/calliope-project/calliope/pull/569?src=pr&el=tree&filepath=src%2Fcalliope%2Fbackend%2Fparsing.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=calliope-project#diff-c3JjL2NhbGxpb3BlL2JhY2tlbmQvcGFyc2luZy5weQ==) | `96.99% <100.00%> (+0.10%)` | :arrow_up: | | [src/calliope/backend/pyomo\_backend\_model.py](https://app.codecov.io/gh/calliope-project/calliope/pull/569?src=pr&el=tree&filepath=src%2Fcalliope%2Fbackend%2Fpyomo_backend_model.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=calliope-project#diff-c3JjL2NhbGxpb3BlL2JhY2tlbmQvcHlvbW9fYmFja2VuZF9tb2RlbC5weQ==) | `98.11% <100.00%> (+0.03%)` | :arrow_up: | | [src/calliope/backend/where\_parser.py](https://app.codecov.io/gh/calliope-project/calliope/pull/569?src=pr&el=tree&filepath=src%2Fcalliope%2Fbackend%2Fwhere_parser.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=calliope-project#diff-c3JjL2NhbGxpb3BlL2JhY2tlbmQvd2hlcmVfcGFyc2VyLnB5) | `98.18% <100.00%> (-0.01%)` | :arrow_down: | | [src/calliope/backend/expression\_parser.py](https://app.codecov.io/gh/calliope-project/calliope/pull/569?src=pr&el=tree&filepath=src%2Fcalliope%2Fbackend%2Fexpression_parser.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=calliope-project#diff-c3JjL2NhbGxpb3BlL2JhY2tlbmQvZXhwcmVzc2lvbl9wYXJzZXIucHk=) | `93.73% <50.00%> (-0.27%)` | :arrow_down: |