Automatically download road network if it doesn't exist

[nreinicke]

Right now we require a road network file as an input to a simulation. We could check to see if a road network file is specified and then, if not, we could try to just download the road network that's needed from a geofence file (I know we just talked about removing it but I think it could make sense in this case, possibly renaming it to something like extent).

[robfitzgerald]

something like

if road_network is not None:
  # read network file
elif extent is not None:
  # call osmnx with extent
else: 
  throw IOError("some useful explanation about networks and extents here")

bearing in mind, OSMNx should remain an optional dependency of HIVE.

[nreinicke]

Closed by #175