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.
e.g. if MedicalEntity were moved into 'health', we'd express this as
We could also represent this as
... but the former is fine and allows easy query of the extension within which a term is "based".
Updated release notice for #655 and #429