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

* incl. new init_internal_schema.sql script (+ generation via code)
I'd suggest to make one more refaktoring step and search and replace "schemaAttributePathInstance*" (case sensitive search to avoid renaming the class) by "schemaAPInstance*" for at least a bit more readability. To be consistent we could also do this for "mappingAttributePathInstance". Especially when even long names are constructed like schemaAttributePathInstanceAttributePathAttributes .... this may be critical. The meaning of AP can easily be derived in an IDE by looking at the type then.