Support loading in multiple models for assembly visualization

[lf94]

It would be nice to be able to load in several meshed SDFs into the same 3D space to visualize assembling them together.

Probably will involve similar metadata as with annotations on libfiveValue objects.

[lf94]

Proposal: Viewer.upload(id, model) will upload the mesh, but only Viewer.instance(id, position, orientation) will place it in the scene. We can have a new Viewer.show(model, position, orientation) to combine the two for single models.