Add spell checking; fix spelling mistakes

[brynpickering]

Fixes #88

Summary of changes in this pull request:

• Add codespell pre-commit hook.
• Check spelling to have the hook pass (we weren't doing too badly).

Not spell checking Jupyter Notebooks due to this issue.

Reviewer checklist:

• ~[ ] Test(s) added to cover contribution~
• ~[ ] Documentation updated~
• ~[ ] 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 (4e70191) 95.00% compared to head (c94a3c5) 95.00%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #530 +/- ## ======================================= Coverage 95.00% 95.00% ======================================= Files 24 24 Lines 3301 3301 Branches 705 651 -54 ======================================= Hits 3136 3136 Misses 96 96 Partials 69 69 ``` | [Files](https://app.codecov.io/gh/calliope-project/calliope/pull/530?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/expression\_parser.py](https://app.codecov.io/gh/calliope-project/calliope/pull/530?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=calliope-project#diff-c3JjL2NhbGxpb3BlL2JhY2tlbmQvZXhwcmVzc2lvbl9wYXJzZXIucHk=) | `97.14% <ø> (ø)` | | | [src/calliope/core/model.py](https://app.codecov.io/gh/calliope-project/calliope/pull/530?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=calliope-project#diff-c3JjL2NhbGxpb3BlL2NvcmUvbW9kZWwucHk=) | `94.70% <ø> (ø)` | | | [src/calliope/exceptions.py](https://app.codecov.io/gh/calliope-project/calliope/pull/530?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=calliope-project#diff-c3JjL2NhbGxpb3BlL2V4Y2VwdGlvbnMucHk=) | `94.87% <ø> (ø)` | | | [src/calliope/postprocess/postprocess.py](https://app.codecov.io/gh/calliope-project/calliope/pull/530?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=calliope-project#diff-c3JjL2NhbGxpb3BlL3Bvc3Rwcm9jZXNzL3Bvc3Rwcm9jZXNzLnB5) | `90.32% <ø> (ø)` | | | [src/calliope/preprocess/time.py](https://app.codecov.io/gh/calliope-project/calliope/pull/530?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=calliope-project#diff-c3JjL2NhbGxpb3BlL3ByZXByb2Nlc3MvdGltZS5weQ==) | `88.57% <ø> (ø)` | |