Simplified one of the gtfs functions to calc distance between segs to use two
GTFS stop objs, instead of stop_time objs.
Moved the create network topology stops from existing GTFS to a more general
name 'create_network_topology_from_existing_gtfs.py', to represent that it
will now calculate and save segment and route topology as well.

The framework for calculating such topology is in place :- just the guts of
the algorithm are missing.