Removal of index.constraint field, and DDLGenerator()

The index.constraint field is superfluous, and creates unnecessary noise. The field is removed, to check if an index is connected to a foreign key, a new method is added, which checks the presence of the specific foreign key in a schema. 

DDLGenerator() is (and was) broken. Depending tests are replaced by assert statements, to verify the modified content of an AIS.
Is needed to drop hidden PK, DropIndexesIT fails otherwise