fixes #241

The grazingAltitudeLocation function returned the closest point to the sphere not on the sphere which was intended I believe, so I changed it to find that.  

There are other little focus weirdnesses that we should just get to in the refactor.
This is trying to find the surface point "under" the grazing altitude location, right?  `Ellipsoid` has methods for that: `scaleToGeocentricSurface` and `scaleToGeodeticSurface`.  The former moves down toward the center of the Earth (as you're doing here).  The latter moves down along the geodetic surface normal (which is closer to what we usually consider down).