Updated this script to use the appropriate seg speed model.
Updated the MultipleTimePeriodsPerRouteSpeedModel to read the speeds from a
saved CSV file, rather than extracting from GTFS on the fly each time.

Necessitated adding func to read back in a CSV file of speeds in gtfs_ops.py -
read_route_speed_info_by_time_periods() - and a few more interface functions
to setup the speed model at the beginning of processing each route etc.