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

* incl. new init_internal_schema.sql script (+ generation via code)
Note (more for myself, will check on Monday): There are a lot of commits concerning only formatting. Is this formatting inline with our latest formatting rules, or do we need to run the formatting? (I think when I last tried the variant with tabs was still used (though I don't like it ;) .