Make alerts sensitive to movement direction of storm

[Lee-Carre]

Related to #156.

Alerts fire regardless of factors other than radius / distance.

Add the possibility (with a configurable toggle) to trigger an alarm only if a storm is approaching.

When a storm is retreating, or simply passing by, that's less of a concern than one which is incoming.

[jaybeezcz]

Oh, yes, that would be very interesting.

[jaybeezcz]

Maybe, it could be an idea, to make the alerting easier by:
Slim down the protocol

• just use short locations codes like GooglePlusCodes for regions, no calculating of whole GPS-Coordinates. G+Codes would be exact enough. (In fact, they are still too exact, i think filtering on RegionCode before the "+-sign" should be ok)
• calculating prediction tracks serverside, not clientside
• maybe store predictions like "regioncode","probably_earliest_ETA", so the App just has to poll these 2 informations
• maybe additional field "severity" for separate warn levels (e.g. from "Maybe lil´bit windy" to "you´ll gonna die shure") depending on the amount of lightnings or smth else.

I´m very interested in this topic, because it would be a live-saving feature in nature, hiking, climbig, everything where you are higly exposed to thunderstorms ...

Bye and Greetz