Allow NIFTI reader and writer to handle planar RGB files.
The NIFTI format uses packed RGB, but the older Analyze format, in its earliest incarnations, used planar RGB. For the sake of compatibility with these old Analyze files, or with NIFTI files that were created from them without a planar-to-packed conversion, a PlanarRGB option has been added to the vtkNIFTIImageReader and the vtkNIFTIImageWriter.