feat: add basic support for location_groups

[hbruch]

This PR adds basic support for location_groups, a subset of the GTFS-Flex specification, which is now integrated in the GTFS spec.

I.e, it

• does not list location_groups and location_group_stops as unknown files and validates their columns
• does not list stops as unused, when they are referenced in a location_group
• lists the number of location_groups in the stats

Note: this is WIP. Currently only UNSORTED StopTimesDaoMode is supported. I'd nevertheless already get some review feedback of you, @laurentg.

Still to do for the minimal implementation:

• check for existance of location_group_ids used in stop_times and location_group_stops
• refactor naming of PackedUnsortedStopTimes DEPTIME_MASK, ARRTIME_MASK, DEPTIME_SHIFT, ARRTIME_SHIFT (or introduce new constants with same values but START_WINDOW/END_WINDOW prefix(?)
• implement packing also for PackingStopTimes or fall back to PackingUnsortedStopTimes in AutoSwitchStopTimesDao
• Code formatting (README's developer guide could be extended to include expected formatting)

Additional checks that could be added:

• endPickupDropOffWindow times are greater or equal to startPickupDropOffWindow
• conditional required/forbidden values in stop_times, if location_group_id is set/unset
• ...

[laurentg]

Many thanks, excellent!
Looks good to me so far. Nice addition anyway.