Add linkage/mapping between vocabularies

[mortenwh]

The thesaurus name can be interpreted as meaning a translation between different standards, i.e., that in the end we will connect, e.g., the cf standard names to the gcmd science keywords.

From Cambridge Dictionaries Online: thesaurus is a ​type of ​dictionary in which words with ​similar ​meanings are ​arranged in ​groups

Consequently, we should have this linking/mapping between vocabularies in this repository. However, it may already exist - see: https://marinemetadata.org/community/teams/ont/vocharmony/vocmapping

[mortenwh]

Seems the mappings that we look for are available from http://www.bodc.ac.uk/products/web_services/vocab/. See also the NERC Vocabulary Server documentation

[mortenwh]

The latest version of the CF standard names is always available from a static url http://vocab.nerc.ac.uk/collection/P07/current/. That list also contains links to same and related definitions on other sites (that's how I found https://mmisw.org/ont/api/v0/ont?format=rdf&iri=http://mmisw.org/ont/cf/parameter used to solve #35, but the NERC list lacks the units entry for the CF variables, so I changed to using the MMI list). If we decide to use the NERC server, we can easily connect vocabularies but it requires also some effort to follow different "same as" links to get information such as the units of CF variables.