ENH: Removal of VTK 4 Backwards Compatibility Superclasses.

Removed vtkSource, vtkProcessObject and subclasses.
http://www.vtk.org/Wiki/VTK/VTK_6_Migration/Overview#Removal_of_VTK_4_Backwards_Compatibility_Superclasses
http://www.vtk.org/Wiki/VTK/Tutorials/New_Pipeline#Converting_an_Existing_Filter_to_the_New_Pipeline