Add a max elevation configuration value to ground stations

We already support a minimum elevation/horizon config for ground stations (to avoid trees, buildings, etc.). We should consider adding a maximum elevation config for azimuth-only tracking stations.

See also https://community.libre.space/t/can-a-simple-tv-rotator-be-used-azimuth-only/2213/ and http://aprs.org/LEO-tracking.html