Read files of a zipped GTFS feed and load them to memory as a list of data.tables.
It will load the following files: "shapes.txt", "stop_times.txt", "stops.txt", "trips.txt",
"agency.txt", "calendar.txt", "routes.txt", and "frequencies.txt", with
this last four being optional. If one of the mandatory files does not exit,
this function will stop with an error message.