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

* incl. new init_internal_schema.sql script (+ generation via code)
hm, dunno. I thought everything re. the new sub schema alignments in our internal schema shoudl be fine now. At least this was my intention. I think that your commit re. the new internal schema should be part of this MR, or? (you may also have a look at the init_internal_schema.sql script)