Some math updates and support for internet based altitude correction

These commits fixed some math errors I found, and added altitude correction via Google Elevation

* fixed incorrect lat/long distance calculation: it was causing incorrect glide ratio display on my xperia active
* fixed NaN errors in vertical speed (kills the vario and vs info)
* fixed altitude correction causing the vario to beep unnecessarily
* added google elevation query whenever altitude is set
* added UI display of the current alititude calibration (none, gps, google elevation)