Add a standard way of providing metadata about subsets

[cmungall]

Depends on

• 80

Please read that first.

There is a need for describing consistent metadata about subsets; e.g

https://github.com/geneontology/go-ontology/blob/master/src/ontology/subsets/go_subsets_metadata.yaml

Most of these can be accommodated by existing properties; but to indicate the purpose of a subset we would need a new enumeration of URIs

[matentzn]

No objection from me, what do you want added concretely?