Add a filter setting for wind stations by elevation

[lhaeger]

Windy displays reported wind in a simple on/off scheme. It would be very useful - especially in the mountains - if the stations displayed could be filtered by elevation, similar to the display of the wind layer.
Use cases:

• comparing forecasts with reported wind: forecast is for a specific height, being able to filter the reported wind layer to a similar height would make it easier to compare both and determine which model if closest to reality at the moment.
• choosing a takeoff for paragliding/hanggliding: most takeoffs in a given area are similarly high up but can only be used in certain wind directions. Filtering to the approx. takeoff elevations removes irrelevant but distracting data from the map, which is usually the majority of weather stations since most of them are located in the valleys and only a few higher up the mountains.
• researching flyability for paragliding: paragliders fly relatively slow and valley winds can get a lot stronger than that sometimes. Filtering reported wind to valley floors would help estimate if the conditions are likely to stay within a safe limit over the course of the day, or could potentially become dangerous.

Filtering by elevation is especially helpful in Windy, since the elevation of a weather station is not visible on the map (also not on mouse-over) but only after selecting it and then swiping to the left for the station details on the bottom of the screen. Try that for an area within 2 hours drive to choose your flying destination for the day and you'll see how much easier it would be with filtering capabilities. :-)