Add script to convert CMIP6 to CMIP7

[lewisjared]

Description

Adds scripts/create-cmip7-datasets.py

Checklist

Please confirm that this pull request has done the following:

• Tests added
• Documentation added (where applicable)
• Changelog item added to changelog/

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

Files with missing lines	Coverage Δ
...te-ref-core/src/climate_ref_core/cmip6_to_cmip7.py	87.41% <ø> (+0.48%)	⬆️

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