Add a systematic approach to traversing VTK's renderables

The basic idea is a uniform means to traverse the various
instances that define the visible scene in VTK, inspect their
state, and do arbitrary operations (build, synchronize and
render thus far) from them.

Interactive rendering is the primary use case. Here we traverse
and make calls to some external API to draw the scene (e.g.
OSPRay). We could potentially write out input files for
other packages (e.g. exporters) or do particular transformations
with this too.