PPMN — (Structural) split out types from instance modeling

Many people will want to use PPMN just for tracking occurrences (which is the level that PROV works at). Understanding how to do this is made a lot more complex by the fact that all the metamodels are presented along with the type/process level information.
Proposal: have the instance/occurrence level information in their own self-contained metamodels and the Type extensions separate.