Add validation: Split/joined trips with incorrect drop-off/pickup types

[hbruch]

Common train operations involve two trains that are joined at a station and then continue the journey as one train, or one train that is split at a station into two trains headed in different directions. Model joining and splitting in GTFS with two separate trips, one for each lineup of vehicles. Use pickup and drop off restrictions to prevent routing results that show duplicated trips for the shared part of the trip.
(Transit Partners Help)

A validation should check this.

I'll submit a PR shortly.