Remove column supertype_id of table datatype

[clausnagel]

It is null for all datatypes currently. What was the purpose of this column?

[thomashkolbe]

I think we have introduced this to have the possibility to represent subclassing for data types. We currently might not have data types in CityGML 3.0 that are derived from other data types, but this could happen in ADEs.

[clausnagel]

Ok, in this case the foreign key constraint on supertype_id is incorrect:

CONSTRAINT datatype_supertype_fk FOREIGN KEY (supertype_id)
  REFERENCES citydb.objectclass (id) MATCH SIMPLE
  ON UPDATE CASCADE
  ON DELETE NO ACTION

The foreign key is currently defined to reference objectclass but shall rather reference datatype itself.