Make a 3D graph class

[sebjameswml]

This should be either an extension of GraphVisual to handle 3D or a separate class.

[optseb]

Or, (as discussed with @fabien-colonnier ) a 'collection' class which collects several individual VisualModels that are intended to be rendered in the same location as a little group (here it would be TriaxesVisual + ScatterVisual or TriaxesVisual + HexGridVisual etc)