Just adding some useful constants and a utility func to gtfs_ops.py
Added in create_network_topology_from_existing_gtfs.py the ability to specify
via a CSV file, for certain lines specific stops that the full-stop building
algorithm should start at.

This was because completely abstract algorithms for constructing these were
in a way that really 'made sense' in terms of Melbourne's train network was
really tricky (perhaps impossible) in certain cases, like Belgrave.

Required adding options in certain functions in route_segs.py too.