Allocating terms to the extension they are defined in

[RichardWallis]

Add an isPartOf property to term definitions in RDFa data files (defaults to 'core').
In code use this to identify homeLayer for a node.
Error check for a term being defined as part of a unknown extension, or defined in more than one.
This will not prevent a term being mentioned in more than one extension.

[danbri]

e.g. if MedicalEntity were moved into 'health', we'd express this as

• http://schema.org/MedicalEntity isPartOf http://health.schema.org/

We could also represent this as

• http://schema.org/MedicalEntity someRelationshipHere http://health.schema.org/MedicalEntity

... but the former is fine and allows easy query of the extension within which a term is "based".

[danbri]

/cc @vholland