First version towards time-dependent speed-varying timetables:
 * Added error checking when detecting segment order.
 * renamed 'config' dictionary to 'mode_config' for clarity.
 * Added a --usesegspeeds parameter to allow enabling
   speeds read from segments, not just a global value of zero.
   (off by default, have not yet tested it works).
Have now got basic time-dependent GTFS going if schedule defines peak and
off-peak periods, and network topology timetable has appropriate fields with
values enabled.

Is off by default, run create_gtfs_from_basicinfo.py with the
--usesegspeeds=true flag to actually use these speeds when creating timetable.

Otherwise, it keeps with existing behaviour of using a constant speed over
entire network defined per-mode.