sub schema (SchemaAttributePathInstance) domain model extension + test re-factoring

* incl. new init_internal_schema.sql script (+ generation via code)
What is the benefit of using Collection here instead of List? Schema will return ordered APInstances, since it returns the values from a linked map impl., but making the order of the collection explicit may "prevent" (or better discourage) developers from ignoring this order in further transformations. If there is no strong reason against a List, I'd change this and also change the return type of getAttributePaths to List.