Add license restriction to iso19139

[frafra]

It would be nice to be able to specify the license under gmd:identificationInfo/gmd:MD_DataIdentification/gmd:resourceConstraints/gmd:MD_LegalConstraints/gmd:useLimitation.

https://github.com/geopython/pygeometa/blob/c8d4f49146828dd03ca8a1a4c1acdc3d94a187f0/pygeometa/schemas/iso19139/main.j2#L242-L244

[tomkralidis]

I'd lean towards something like (in the MCF) the below proposal:

identification:
    license:
        name: CC-BY-4.0
        url: https://spdx.org/licenses/CC-BY-4.0.html

where the license object must define at least one of name or url. The above can then be applied in a more generic manner to numerous supported metadata formats, including ISO of course.

@frafra thoughts?

[frafra]

It seems great to me :)

[tomkralidis]

@frafra: implemented in #212. Can you rebase/update #206 accordingly? Thanks