Add op to "side-load" USD stage for rendering

[nrusch]

Being able to render a USD stage in Hydra without passing it through Nuke's 3D system would be quite advantageous in a fully-realized workflow. This functionality is currently implemented in a barebones fashion on the HydraRender op for testing purposes, but I think it's worth a dedicated op.

I think this op should derive from the same intermediate AxisOp subclass mentioned in #3.

[nrusch]

I've implemented an initial version of this op, but I'm going to leave this open, since I still want to add support for time remapping and population masking.