Extend default_stations method

[krisgesling]

Currently default stations are defined on a per country basis. If multiple stations are defined and these are geographically based we could provide the most localized station available for each user.

Eg #96 is a station for the state of Hessen, Germany.
Hence if a user is located in Hessen, they should receive the Hessen station, rather than the national default.
Similarly if there was a Berlin station, that should be the default for users from that city.

As always these can be overridden in settings and other stations can be requested by name.

Need to consider the best way for defining this for each station, and then how to select based on that definition.