define data model identifer(s) for use in tap_schema #18

The current standard defines a versioned ivo identifer for use in TAPRegExt records, but more recent usage has leaned toward putting the model identifier into tap_schema.tables using the utype column.

• consider (again) how best to version identifiers so they are queryable in a sane way
• consider identifier usage with respect to extensions
• maybe: define a standard INFO element to output the identifier in query responses