HoudiniScene node() and embedded()

I've added a binding for node(), which gives access to the underlying Houdini node. I've also added a new method called embedded() which returns a bool indicating if the scene is pointing to hierarchy embedded inside a SOP. These will both be of use to Paulo for his inspection UI.

Note that we aren't currently returning a PyObject* directly, because Houdini only provides a mechanism for that using swig, which we don't build. I've requested that SideFx provide a swig-free approach, and if they do, we should fix this up at that time.