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
Adding sensor boundary line width variation.
Adding sensor boundary intersection property.
Restored correct default intersection width.
Updating CHANGES.md file for pull request.
Improved intersectionWidth documentation.

Added "approximate pixel" as size description.
Merge pull request #929 from fstoner/master

Improvements to custom sensor volume fragment shader.