message: Removed rtk::VectorImage from code/rtkForwardDifferenceGradientImageFilter.h
  as suggested in issue 89
diff: namespace rtk
  {
  -
  -template <typename TPixelType, unsigned int VImageDimension > class VectorImage;
  -
  -
  /** \class ForwardDifferenceGradientImageFilter
  * \brief Computes the gradient of an image using forward difference.
  *
  