Fix geoidseparation

The GPS returns data referenced to the geoid (either EGM1996 or EGM2004, depending on the GPS chipset), which was previously converted to reference to the ellipsoid. Now with this PR, the 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