Check that there is at most one StopTimeUpdate for each stop in a trip

[barbeau]

For TripUpdate. This would generate an error. See https://groups.google.com/d/msg/gtfs-realtime/qNc7ButQbW8/tDwanvClBQAJ.

[barbeau]

Note that there are a few existing rules/issues related to this, but that don't cover this issue exactly:

• E002 - Unsorted stop_sequence (for stop_time_updates that include stop_sequence, and https://github.com/CUTR-at-USF/gtfs-realtime-validator/issues/159 for stop_time_updates that don't include stop_sequence)
• E036 - Sequential stop_time_updates have the same stop_sequence
• E037 - Sequential stop_time_updates have the same stop_id
• E046 - GTFS-rt stop_time_update stop_sequence and stop_id do not match GTFS

So it seems that a good implementation for a new rule here would be to implement a HashMap used to check that each stop_sequence appears only once for all stop_time_updates in a trip (to be implemented in StopTimeUpdateValidator). We can do the same with stop_id, but only for routes that don't include loops that visit a stop_id more than once (i.e., if the trip_id key doesn't exist in GtfsMetadata.getTripsWithMultiStops() HashMap). It seems like this could actually replace E036 and E037 if it covers those scenarios.