Improve station's scope logic

[Fabi755]

At the current version station prices are compared with all stations in radius a radius of 20 kilometers.

In general this works well, but the real street way is very different based on roads and city.

Instead for calculating every time the stations around, the comparing-reference-stations should stored in a database table. This should updated regulary or automatically for new stations.

It should also be possible to finetune this manual and change them by our (future) admin panel.

The grouping of referenced stations may be pure math, or by street distance, or by drive-duration. But this is in the end fine tuning and not important to define yet.