ENH: VTK6 migration, add vtk6 into build system.

Added vtk6 to build system and give users a build option between vtk5 and vtk6.
ENH: VTK6 migration, replace SetInput() and SetSource().

Replacement of SetInput() with SetInputData() and SetInputConnection(), same as SetSource().
http://www.vtk.org/Wiki/VTK/VTK_6_Migration/Replacement_of_SetInput