Add new horizon culling point computation.

This one is courtesy of Frank Stoner.  It is faster and can be used with
an ellipsoid, not just a sphere.
EllipsoidalOccluder improvements.
Use new horizon occlusion point computation.
Fix horizon occlusion point problems.
Remove unused Occluder from CentralBodySurface.
Fix problems with small central bodies.
Use heights when computing occlusion point.

Also tests and minor algorithm improvements.
Update CHANGES.md.
Fix warning.
Remove unused require.
Merge remote-tracking branch 'origin/master' into horizonCullingPoint

Conflicts:
	CHANGES.md
Merge pull request #749 from AnalyticalGraphicsInc/horizonCullingPoint

Improve horizon occlusion point computation