feat: warn when trip_headsign matches intermediate stop name

[PatrickSteil]

Adds a new validation rule TripHeadsignValidator that warns when a trip's trip_headsign matches the stop_name of an intermediate stop (i.e., any stop that is not the last stop of the trip). This can cause confusion for passengers boarding after that stop, as the headsign suggests the vehicle is heading somewhere it has already passed.
The notice TripHeadsignMatchesIntermediateStopNotice is emitted as a WARNING and includes:

• the offending trip_id and its row number in trips.txt
• the trip_headsign value
• the stop_id and stop_sequence of the intermediate stop matching the trip_headsign
• the stop_id of the actual last stop of the trip

NoticeFieldsTest has been updated to register a new notice field name introduced: tripHeadsign.

Note that if a trip visits the last stop multiple times, then no warning is shown.

The following screenshot shows a real-world example of the official Swiss GTFS feed, where a trip passed through St. Gallen, and continues to Winterthur. But the trip_headsign shows St. Gallen.

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[PatrickSteil]

In this PR patrickbr/gtfstidy#41, i posted the same example but with a screenshot of the trip's route, as well as implementing a fix by overwriting the stop_headsign.