Implement train splitting / joining as GTFS transfers

[miklcct]

https://github.com/google/transit/pull/303 adds the ability to specify same-vehicle transfer in GTFS. Splitting and joining trains within a passenger service can be specified with transfer_type = 4 for the constituent portions.

[linusnorton]

Okay, so rather than have the new services, just populate transfers.txt with same vehicle transfers? Seems like a good idea to me.

[miklcct]

For a service A-B-C splitting off a B-D portion, the specification models it as a A-B service, B-C service and B-D service, which the A-B service has an in-seat transfer with the both B-C and B-D services.

[linusnorton]

Okay, the ATOC/DTD schedules do ABD + association record + BC. So that'd need to become AB, BC and BD (with appropriate transfers)