Add schema updater

[brynpickering]

Fixes issue of adding new parameters but not getting any of the benefits of having default data etc. stored in the model schema. This is useful for our separate pathways repo.

Summary of changes in this pull request:

• Add schema updater for the model definition schema specifically.
• Add schema reset button to allow a user who has messed up their schema with weird things to return to a blank slate (would require a reload of Calliope otherwise).

Reviewer checklist:

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

[codecov[bot]]

Codecov Report

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

Comparison is base (0e12b05) 95.83% compared to head (df65db2) 95.85%. Report is 1 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #568 +/- ## ========================================== + Coverage 95.83% 95.85% +0.01% ========================================== Files 24 24 Lines 3604 3619 +15 Branches 787 722 -65 ========================================== + Hits 3454 3469 +15 Misses 86 86 Partials 64 64 ``` | [Files](https://app.codecov.io/gh/calliope-project/calliope/pull/568?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/util/schema.py](https://app.codecov.io/gh/calliope-project/calliope/pull/568?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=calliope-project#diff-c3JjL2NhbGxpb3BlL3V0aWwvc2NoZW1hLnB5) | `92.20% <100.00%> (+1.88%)` | :arrow_up: |