model trip_id for cyclical public transport routes

[namdre]

when modelling a cyclic route it may be useful to know which part (or iteration) of the cycle the vehicle is currently in. In VDV this is called "FRT_ID" in GTFS its "trip_id".

To achieve this a new optional stop attribute 'tripID' should be supported. When reaching such a stop, the vehicle param 'tripID' is updated to that of the stop