ODX - transfers / multiple vehicle trips

[antrim]

Current draft's definition of rider_trip.trip_id:

The trip_id field contains the ID of the trip associated with the unique rider, if this is known. If trip_id is empty, then the rider may have taken one of several trips, or several possible chains of trips, to travel between the origin and the destination.

"Origin-destination-transfer" (ODX) data is available from onboard passenger surveys, fare collection systems that require tap off, can be inferred in the case of "tap-on only" fare systems. It may also be available through mobile ticketing apps, though I am uncertain on this. It would be useful for GTFS-ride to be able to specify a rider trip across multiple vehicle trips.

One option: Add a new file, ride_segment.txt (see issue #21) or rider_trip_segment.txt, which could associate rider trips to vehicle trips, i.e. with fields for rider_trip_id, trip_id (foreign key to trips.txt), boarding_stop_id, alighting_stop_id, etc.