New transitfeed GTFS operation funcs:
* create_base_schedule_copy()
* copy_stops_with_ids()
* get_stop_ids_set_used_by_selected_routes()

Also changed the argument order and type for a func to be more consistent, is
now:
* get_all_stop_ids_used_by_route(schedule, route_id)
  (Args used to be get_all_stop_ids_used_by_route(gtfs_route, schedule) )
New argument allowed for script subset_trips_by_route.py :-

  --output_rem=OUTPUT_REM
    (Optional) Path of output GTFS file to save "remainder" routes, not in
    selection, to (Should end in .zip).

Refactored to use relevant new funcs in gtfs_ops last commit.