When tosca relationship types are added it must be possible to specify the model they belong to.

Adding a type to a single model only will be supported

Story includes updating the APIs to accept the model the relationship type should be associated with, all the way to creating an edge between the vertex for the relationship type and the vertex for the relevant model in the database.

It shall be valid for no model to be specified, in which case the type shall not be associated with any model (i.e. existing functionality maintained)