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.
Did you mean the variable "schema"? This needs to be set to check whether a index is connected to a foreign key (ln 1189 and ln 1236), once per table.