`license_id` validation

[znichollscr]

The motivation

We missed checking for license_id with the land-use data, which made the views look silly: see PCMDI/input4MIPs_CVs#136

The proposed solution

• add check for license_id. It must have a value of CC BY 4.0 at present (although we should just get the accepted values from the CVs spec)

Alternatives

Additional context

PCMDI/input4MIPs_CVs#136