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

* incl. new init_internal_schema.sql script (+ generation via code)
I guess the List was changed to a collection here (order not guaranteed), since we expected SchemaAPInstances to have a field order. Looking at the current SchemaAttributePathInstance impl I noticed that I skipped this, so we will probably want to add a field for order like in the mapping attribute path instance. However, shouldn't we rather use a list to store the order instead of reconstructing it each time we need it from the order values?