OPENJPA-2305: Restore the inheritance hierarchy of the generated canonical metamodel to that of the domain model due to spec compliance, but flatten the attributes so that an inherited attribute uses generic parameter of owning class type instead of the declared supertype

git-svn-id: https://svn.apache.org/repos/asf/openjpa/trunk@1451369 13f79535-47bb-0310-9956-ffa450edef68