Refactored the GTFS creation scripts so that:-
 * Variables and structs purely to do with specifying a timetable and info
   about mode speeds etc is now in a separate Python file
   (mode_timetable_info.py)
 * Similarly, global variables defined for accessing the network topology
   shapefile, in effect the 'data model', are in a separate file
   (topology_shapefile_data_model.py)

These were all previously in create_gtfs_from_basicinfo.py, but have been
moved, feel this is now a bit cleaner and better cohesion as the others
scripts import only what is necessary, not everything in the main script.
Is a move towards a more abstracted data model too, in case e.g. we wanted
to move to PostGIS database and Object-Relational Model for storing this data
in future rather than shapefiles.

Also, added a simple 'plotfunc.py' script to plot the speed-vs-distance from
CBD function now used for bus peak speeds, for validation purposes.