Support gtfs feeds without individual stop times

[Rikuoja]

Describe the bug
Some GTFS feeds (notably Madrid metro) do not contain all the individual stops during the whole day. Instead, the stop frequency at each stop can be calculated from line frequencies from frequencies.txt.

To Reproduce
Steps to reproduce the behavior:

1. Use Madrid Metro GTFS feed URL https://transitfeeds.com/p/consorcio-regional-de-transportes-de-madrid/742/latest/download
2. Wonder why all the metro stops have only 1 or a few departures during the whole day.

Expected behavior
Stop frequency should be calculated from line frequencies in frequencies.txt, if that file contains data. Stop times should only be used if frequencies.txt is not present or is empty.