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.
This should probably go away. If you remove the special handling of PRIMARY does anything fail? If so, it needs to switch to getPrimaryKey() directly.