Add `title` to math var and expr components

[brynpickering]

Fixes #582

Chosen "title" as it fits with the YAML/JSON schema syntax.

Summary of changes in this pull request

• Variables and global expressions get a "title" key that will be stored in the attributes of the result dataarrays
• Parameters already have a "title" key in the schema, this is also now passed to the attributes of the input dataarrays

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:

Project coverage is 95.34%. Comparing base (872978d) to head (d2c8e99). Report is 18 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #603 +/- ## ========================================== - Coverage 95.85% 95.34% -0.51% ========================================== Files 24 24 Lines 3619 3653 +34 Branches 788 734 -54 ========================================== + Hits 3469 3483 +14 + Misses 86 82 -4 - Partials 64 88 +24 ``` | [Files](https://app.codecov.io/gh/calliope-project/calliope/pull/603?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/603?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==) | `98.08% <100.00%> (+0.17%)` | :arrow_up: | | [src/calliope/backend/latex\_backend\_model.py](https://app.codecov.io/gh/calliope-project/calliope/pull/603?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==) | `96.32% <ø> (-1.48%)` | :arrow_down: | | [src/calliope/backend/parsing.py](https://app.codecov.io/gh/calliope-project/calliope/pull/603?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==) | `96.01% <100.00%> (-1.76%)` | :arrow_down: | | [src/calliope/backend/pyomo\_backend\_model.py](https://app.codecov.io/gh/calliope-project/calliope/pull/603?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==) | `95.59% <ø> (+0.17%)` | :arrow_up: | ... and [1 file with indirect coverage changes](https://app.codecov.io/gh/calliope-project/calliope/pull/603/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=calliope-project)