Add license validation and handling

[znichollscr]

The motivation

As a data producer, I want licence information to be filled out for me rather than having to type lots of stuff.

The proposed solution

Use the CVs to handle licence stuff.

Alternatives

Additional context

It looks like the CMOR tables are going to simplify this considerably, hence this is blocked until they are settled.

https://github.com/PCMDI/mip-cmor-tables/blob/jsonld/JSONLD/license/cc-by-sa-4.0.json

[durack1]

Yep, so the default license is CC-BY-4.0, with a couple of variants also allowed - the variants we had included in CMIP6 are included at https://github.com/WCRP-CMIP/CMIP6_CVs/blob/main/CMIP6_license.json, with a strong preference to avoid the NC and SA variants, as this complicates downstream data use