Fix geoidseparation

The GPS returns data referenced to the geoid, which was previously converted to reference to the ellipsoid. Now reported altitude will correspond to WGS-84 data sets which reference altitude above geoid. In particular, Google Earth [seems to use](https://groups.google.com/forum/#!topic/google-earth-browser-plugin/bqM7OjwEzBM) the EGM96 geoid, and [World wind does](http://forum.worldwindcentral.com/showthread.php?35638-World-wind-elevation-model)


Note that the tablet conversion is likely incorrect, or at least insufficient:
http://stackoverflow.com/a/12784563
https://code.google.com/p/android/issues/detail?id=53471