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, removal of deprecated Macro.

These two macros were deprecated in VTK 5.0 that were still supported, but have now been eliminated. These include:
vtkTypeRevisionMacro has been replaced with vtkTypeMacro; vtkCxxRevisionMacro has been removed.
http://www.visitusers.org/index.php?title=VTK_6.0_Upgrade
ENH: VTK6 migration, vtkFloatingPointType is replaced with double.

ref: http://review.source.kitware.com/#/t/1667/
ENH: VTK6 migration, replacement of SetInput().

Replacement of SetInput() with SetInputData() and SetInputConnection(), the same kind of functions include SetInput1(), SetInput2() and SetSource().
http://www.vtk.org/Wiki/VTK/VTK_6_Migration/Replacement_of_SetInput
ENH: VTK6 migration, removal of GetProducerPort().

1)Removal of GetProducerPort() from vtkDataObject.
http://www.vtk.org/Wiki/VTK/VTK_6_Migration/Removal_of_GetProducerPort
ENH: VTK6 migration, removal of Execute, vtkType deprecations.

1) vtkType deprecations
http://www.vtk.org/Wiki/VTK/VTK_6_Migration/vtkType_deprecations
2) Removal of Execute
http://www.vtk.org/Wiki/VTK/VTK_6_Migration/Removal_of_Execute
ENH: VTK6 migration, removal of SetWholeExtent and update extent methods.

1) Removal of SetWholeExtent() from vtkDataObject
http://www.vtk.org/Wiki/VTK/VTK_6_Migration/Removal_of_SetWholeExtent
2) WIP: Removal of Methods for Manipulating Update Extent
http://www.vtk.org/Wiki/VTK/VTK_6_Migration/Removal_of_Methods_for_Manipulating_Update_Extent
ENH: VTK6 migration, change to AllocateOutputData.

Change to AllocateOutputData() in vtkImageAlgorithm.
http://www.vtk.org/Wiki/VTK/VTK_6_Migration/Change_to_AllocateOutputData
ENH: temp