Flex spec upgrade (separate location_id and group_id)

[leonardehrenfried]

Summary:

During the specification process of GTFS Flex the group decided that we don't want to put location and location group ids into the the column stop_id anymore. Instead new columns location_id and location_group_id instead.

This makes the column stop_id optional.

I've also introduced a new method StopTime#getStopLocation that returns the correct entity (stop, location, location group).

cc @gcamp @westontrillium @tzujenchanmbd @eliasmbd

[westontrillium]

Example file for location groups: https://data.trilliumtransit.com/gtfs/auburntransit-ca-us/auburntransit-ca-us--flex-v2-TEST.zip

[sheldonabrown]

Thanks @leonardehrenfried