Use half of the headway to generate time for frequency based trips

[miklcct]

This isn't exactly a bug, but the current code generates time for a frequency based trip which isn't exact times by adding the full headway into the requested time.

For example, if a trip runs every 5 minutes and ends at 23:00, it returns 22:59 if I specify 22:54, and returns no result if I specify 22:55 even if I should be able to catch a service between 22:55 and 23:00.

Also, if an itinerary contains multiple transfers of frequency-based trips, the timing will become unrealistic as the full headway is added to every transfer, while in the real world the expected waiting time is half of the frequency.

I think we need to discuss if we should give an option to generate times for half-headway basis.