Support gdal palette images

Updates vtkGDALRasterReader to load images that use the GDAL pallete/index coloring, typically used to represent information categories. The pixel data are written to point-data scalars, same as other image types. The default name of the scalar array is set to "Categories" (instead of "Elevation", which is the gray-scale default). Color palette data is read from the input image and used to construct a vtkLookupTable stored with the scalar data array. The lookup table is set to indexed-lookup mode. If category names are included with the input image, they are added as annotations to the lookup table; otherwise default annotations are created.