Update zenodo source of gridded weather data

[brynpickering]

Fixes some review comments from @irm-codebase in #390.

Made a specific PR as they are changes that are worth considering in isolation, I believe. Deals with data units, mostly.

I'll deal with the rest of the review comments in #390 directly.

Checklist

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• [ ] CHANGELOG updated
• [ ] Minimal workflow tests pass
• [ ] Tests added to cover contribution
• [ ] Documentation updated
• [x] Configuration schema updated