User is now allowed to pass a Pointer to a function of his own

  User is now allowed to pass a Pointer to a function of his own
      to allow modification of pixel order (i.e. : Mirror, TopDown, )
      to gdcm::FileHeleper, using SetUserFunction(userSuppliedFunction)

      described as : void userSuppliedFunction(uint8_t *im, gdcm::File *f);

      NB : the "uint8_t *" type of first param is just for prototyping.
        User will Cast it according what he founds with f->GetPixelType()
        See ctkgdcmSerieViewer for an example