Add CMIP7 support #2874
Description
Add support for CMIP7 when data becomes available.
- Information: https://wcrp-cmip.github.io/cmip7-guidance/CMIP7/guidance_for_modellers/
- facet, file, and directory names: https://wcrp-cmip.github.io/cmip7-guidance/CMIP7/global_attributes/
- CMOR tables: https://github.com/WCRP-CMIP/cmip7-cmor-tables
- Script to reCMORize some CMIP6 data so it looks like CMIP7 for testing purposes: https://github.com/WCRP-CMIP/cmip7-cmor-tables/blob/main/Simple_recmorise_cmip6-cmip7.ipynb