Pass src_conn and dst_conn to generators when we create-data

[Iain-S]

Begins work on #24

We pass in the src and dst database connections as we construct SSG Generator objects so that the user can either

1. Query an existing dst database table (e.g. as we do for Foreign Keys)
2. Query a src database table (e.g. to get a realistic ratio of M/F patients)

Note that there is no DP or noise at this point.

We also rename the foreign key provider because it is more general than the name suggests.