BUG: Ensure MultiVolume modules work with VTK 5 and 6

This commit updates the modules to check for the version
of VTK (using `vtk.VTK_MAJOR_VERSION`). This is required
because the MacOSX factory is still building against VTK5.

For more details see:

  fedorov/MultiVolumeExplorer@1053c40
  fedorov/MultiVolumeImporter@351226a

From: Andrey Fedorov <fedorov@bwh.harvard.edu>

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23480 3bd1e089-480b-0410-8dfb-8563597acbee