Only apply NoDataValue when it is set in the input image

This branch addresses 2 issues with vtkGDALRasterReader. First, that the handling of the GDAL NoDataValue was not checking whether the input image had set a NoDataValue. Second, that the current reader was doing an implicit cast when comparing the NoDataValue to each pixel value. This caused errors with some GeoTIFF images from a particular government agency, that set the NoDataValue to full double precision when their raster values are floating point. To resolve this, the NoDataValue is now explicitly cast to the input data type.