Phase II of discontinuous Galerkin data processing.

This adds support for rendering discontinuous (and continuous) attributes on the following cell shapes:

Renderings of supported cell types.

Hexahedra	Tetrahedra	Wedges	Pyramids	Quadrilaterals	Triangles

It also supports non-isoparametric elements (where the shape and color attributes do not have the same polynomial order for their parameterization):

Note in the image above that each of the two hexahedra have linear geometry but the color varies quadratically along each coordinate axis of the cell's parameter-space.