Add a StationSet class to datatypes.
Add a StationSet class to datatypes.

get_polling_station_lookup will only be needed when we match stations
to uprns using stations and district polygons. We'll get to that later

DemocracyClub/UK-Polling-Stations,DemocracyClub/UK-Polling-Stations,DemocracyClub/UK-Polling-Stations