feat: "baked in" CMIP6 tables (and maybe CMIP7)

[pgierz]

At the moment, we force the users to give a path to CV_dir and Tables_dir. I find that super, especially as these tables change quite a lot. However, I think it would still be useful to provide a few, well-tagged table and cv versions that we can included as "baked in" references, e.g. the user just needs to write down in YAML:

general:
    cmip_version: CMIP6

This would result in some log like:

Since CV_dir and Tables_dir are not set, we assume you want to use the following:
    * tag X.Y.Z (commit ABCDEF of URL) for Tables_dir
    * tag X.Y.Z (commit HIJKLM of URL) for CV_dir

Or maybe something more machine-friendly in JSON. In any case, a shortcut to well-known, supported versions would be nice.