Add support for xeogl for 3D visualization

[nathancarter]

xeogl is an open-source WebGL-based 3D visualization engine from @xeolabs on Twitter. Scenes can be specified in JSON format.

Because this is not another 2D graphing tool, but rather a 3D scene library, it will be accessible only through CreateVisualization and not through Plot() or any of the other high-level tools. (No need to extend ConvertDataSeriesForTool.)

Adding this will be a significant amount of effort, so it is not high-priority.

Once this is done, add a section to the manual explaining basic use of xeogl and one or two examples of using it from GAP, such as making a polyhedron.

(I'm adding this issue as part of an effort to make my to-do list for this project public.)