COMP: Support VTK6 in CTK tests

1) Support VTK6 in CTK tests
2) use SYSTEM_ZLIB
COMP: VTK6 migration, vtk library python wrapping.
ENH: VTK6 migration, additional replacement of SetInput().

http://www.vtk.org/Wiki/VTK/VTK_6_Migration/Replacement_of_SetInput
ENH: VTK6 migration, miscellaneous changes.
ENH: VTK6 migration, removal of update() from vtkDataObject.

Removal of Pipeline Update Methods from vtkDataObject.
http://www.vtk.org/Wiki/VTK/VTK_6_Migration/Removal_of_Update.
ENH: VTK6 migration, update vtkMRMLVolumeNode and its subclasses.

ImageData to ImageDataPort
ENH: VTK6 migration, update vtkMRMLModelNode and sublcasses.
ENH: WIP: vtkMRMLModelNode API refactoring.