More Pythonic VTK wrapping

[ben.boeckel (Ben Boeckel (Kitware))]

@MicK7

it would be nice to have a pythonic API coherent with networkx or other graph package that would allow plotting the pipeline graph in matplotlib.

Ideally this would be “optional” and live under something like vtkmodules.util.networkx_support (a sibling of numpy_support. I don’t think import vtkmodules.vtkCommonCore should require networkx.