Update math docs and handling of missing data

[brynpickering]

Fixes issue mentioned in #559 w.r.t. information provided in the pre-defined math docs.

Summary of changes in this pull request:

• Parameters are now included in the math docs directly, with cross-refs to the other math components in which they're used.
• Cross-refs have been extended to use of a constraint/variable in the where string of other math components, not only in the math expression.
• Variables/global expressions have defaults which are used to initialise empty array elements. This helps handle situations where a sparsely defined model would lead to NaN values that shouldn't make their way into the LP file. The values are set such that the variable/expression does not impact the optimisation problem if taking its "default" value.
• Updating a Pyomo parameter now looks at the use of the parameter in where strings to decide whether some variables/expressions/constraints need to be re-built. This should be expected functionality (i.e., if updating a parameter turns a NaN input value into a non-NaN value, then the where string should be re-evaluated).
• Parameter units moved to a user-defined schema attribute so we can load them into the math docs. This does mean they are lost from the schema reference page (user-defined attributes aren't picked up there).
• Added extra info to variable/expression/parameter descriptions in the math docs. Namely, default values, unit, and cross-refs.

TODO:

• [x] Either none or only a subset of parameters should show up in the "inter-cluster storage" math docs. Currently, all parameters relavant to base and inter-cluster storage math are shown.

Reviewer checklist:

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

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (4e77873) 95.77% compared to head (69bccd9) 95.83%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #563 +/- ## ========================================== + Coverage 95.77% 95.83% +0.05% ========================================== Files 24 24 Lines 3574 3599 +25 Branches 780 719 -61 ========================================== + Hits 3423 3449 +26 Misses 86 86 + Partials 65 64 -1 ``` | [Files](https://app.codecov.io/gh/calliope-project/calliope/pull/563?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/563?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=calliope-project#diff-c3JjL2NhbGxpb3BlL2JhY2tlbmQvYmFja2VuZF9tb2RlbC5weQ==) | `97.89% <100.00%> (+0.01%)` | :arrow_up: | | [src/calliope/backend/expression\_parser.py](https://app.codecov.io/gh/calliope-project/calliope/pull/563?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=calliope-project#diff-c3JjL2NhbGxpb3BlL2JhY2tlbmQvZXhwcmVzc2lvbl9wYXJzZXIucHk=) | `97.18% <100.00%> (+0.02%)` | :arrow_up: | | [src/calliope/backend/latex\_backend\_model.py](https://app.codecov.io/gh/calliope-project/calliope/pull/563?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=calliope-project#diff-c3JjL2NhbGxpb3BlL2JhY2tlbmQvbGF0ZXhfYmFja2VuZF9tb2RlbC5weQ==) | `97.76% <100.00%> (+1.52%)` | :arrow_up: | | [src/calliope/backend/parsing.py](https://app.codecov.io/gh/calliope-project/calliope/pull/563?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=calliope-project#diff-c3JjL2NhbGxpb3BlL2JhY2tlbmQvcGFyc2luZy5weQ==) | `97.76% <100.00%> (-0.90%)` | :arrow_down: | | [src/calliope/backend/pyomo\_backend\_model.py](https://app.codecov.io/gh/calliope-project/calliope/pull/563?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=calliope-project#diff-c3JjL2NhbGxpb3BlL2JhY2tlbmQvcHlvbW9fYmFja2VuZF9tb2RlbC5weQ==) | `95.41% <100.00%> (+0.45%)` | :arrow_up: | | [src/calliope/backend/where\_parser.py](https://app.codecov.io/gh/calliope-project/calliope/pull/563?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=calliope-project#diff-c3JjL2NhbGxpb3BlL2JhY2tlbmQvd2hlcmVfcGFyc2VyLnB5) | `97.29% <100.00%> (+0.02%)` | :arrow_up: |

[irm-codebase]

Fantastic addition! Adding a list of the constraints where a parameter is used is a really good idea. However, it seems like some math is broken in the new parameters section. Here's an example:

[brynpickering]

OK great, that was a missing $ in the string. If you find any others, do flag them!

[brynpickering]

Chosen a subset of parameters listed in the inter-cluster storage docs. Also updated links in docs to prefer linking to the math docs parameter list rather than the schema. Annoyingly, because the math docs are auto-generated it isn't possible to link directly to the Parameters anchor in the math docs.