Add cmip6 to cmip7 translation

[lewisjared]

Description

Adds functionality to translate an existing CMIP6 dataset to follow the new CMIP7 conventions.
This used the results produced by https://github.com/WCRP-CMIP/cmip7-cmor-tables/blob/main/Simple_recmorise_cmip6-cmip7.ipynb, without needing to pull in the CMOR dependency.

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

❌ Patch coverage is 85.96491% with 16 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...te-ref-core/src/climate_ref_core/cmip6_to_cmip7.py	85.96%	6 Missing and 10 partials ⚠️

Files with missing lines	Coverage Δ
...te-ref-core/src/climate_ref_core/cmip6_to_cmip7.py	85.96% <85.96%> (ø)

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[lewisjared]

@matthew-mizielinski FYI: For the REF I'm planning on making a temporary translation layer to allow us to write out some CMIP7-like files based on your example notebook. It's likely missing a lot of edge cases, but fine for our use-case (monthly datasets from a small subset of experiments).