Metadata - support 'vocabulary' in the <field> element

[CecSve]

Note - this issue is to discuss the best way forward. Not a suggestion for implementation right now.

Regarding: https://dwc.tdwg.org/text/#24-the-field-element

Issue
Currently, the meta.xml file refers to the Darwin Core term for the field definition but does not include information on a resource in case there is a controlled vocabulary in use.

The resource for the field definition example does link to a 'thesaurus' but the information is several clicks away and many users would probably not know how to access it.

Idea for a solution
It might be useful for data users to be able to access information about which fields were interpreted with a controlled value, ideally referring to the vocabulary URI and version, served as SKOS or RDF as suggested by the guide, within the metadata file.

[CecSve]

Relevant vocabulary issue gbif/vocabulary#160

[tucotuco]

I think this is a very important consideration for the new DwCA_v2 publishing model as well. It would be great to declare recommended controlled vocabularies conspicuously in a field meant explicitly for that purpose.