Define semantics for referential integrity

[cmungall]

A non-inlined class range may be intended as:

• a reference to an object in the same document, database, or graph
• a reference to an object in the external document, database, or graph

For external references, the mechanism for deferencing will vary depending on substrate:

• for rdf, the reference MUST be a URI. If the URI is a URL then the URL may be deferenceable by conneg
• for json/yaml docs, there would need to be a bridge from URIs to e.g. JSONPointer
• for sql there would need to be a way to bind URIs to other databases

In all cases some kind of configuration would need to be supplied.

Additionally, the interpretation may be one of:

• the reference MUST be present
• the reference SHOULD be present

Furthermore, the semantics of deleting could be one of:

• no action
• cascading delete