Gave each route a defined prefix char, to avoid hard-coding in other funcs.
 * Fixed a func route_name_from_id() to take in a mode_config to get a
   route prefix, rather than hard-coding as 'R'.
 * Refactored the create_network_topology_segments.py script to also
   create the route_defs CSV, since necessary info was already created
   and loaded.
 * (Involved generalising, moving and re-using some functionality, such
    as code to create route directions.)