Grid material and Ellipsoid primitive

This is my first attempt at creating a Cesium material.

This branch now includes another merged branch with changes to the `EllipsoidPrimative` to make it appear double-sided (to show the far wall of the ellipsoid even when outside).

There are some known depth issues where ellipsoids intersect, but these existed prior to this branch and may eventually be addressed by #607.

This branch used to include logic for adding additional grid lines.  This code has been reverted, as it made overlapping ellipsoids look too crowded.  It is still available for inspection on other branches.