Using implicit surface function for ellipsoid.

Also some minor refactoring of boundary test.:
Simplification of ellipsoid surface function.
Removed ellipsoid ray intersection test.

The test in unnecessary.  It does elliminate a portion of the artifact
that appears when looking at a sensor facet edge on (the portion that
extends past the ellipsoid), but the real fix should be to the boundary
test.
Improvements to boundary test.
Fix for derivatives use in sensor shader.

Earlier code cleanup and refactoring made the artifact problem worse.  But
this change addresses the fundamental problem that use of the derivatives
was causing.
Adding comments to boundary tests for clarity.
Merge branch 'master' into sensorImprovements