Accept some malformed GTFS (lenient mode)

[laurentg]

Accept the most common forms of invalid GTFS (optional, lenient mode), for example:

• CSV data in a sub-directory of the zip (seems rather common in GTFS from GTFS-data-exchange).
• unknown calendar ID in trip (solution: skip the trip)
• unknown trip ID in stop time (solution: skip the stop time or the entire trip)
• unknown stop ID in stop time (solution: skip the stop time or the entire trip)
• missing stop geocoding (solution: default geocode to default place)
• ...