This PR adds support for trip to trip transfers in the googletransit extension.
A new transfer.py class was created to validate the additional fields. Checks are in place for duplicate ids and invalid trip_id or route_id references.
LoadTestCases can now take a GTFSFactory in the .Load() method in order to be able to test extensions.
Tests have been added to check for duplicate IDs and invalid trip_ids.
This PR adds support for trip to trip transfers in the googletransit extension.
More information on Trip To Trip transfers: https://developers.google.com/transit/gtfs/reference/gtfs-extensions#TripToTripTransfers