code: def shift_coordinate_grid(self, x_shift, y_shift, pixel_unit=False):
          
          if pixel_unit is True:
              ra_shift, dec_shift = self.map_pix2coord(x_shift, y_shift)
          else:
              ra_shift, dec_shift = x_shift, y_shift
          self._ra_at_xy_0 += ra_shift
          self._dec_at_xy_0 += dec_shift
          self._x_at_radec_0, self._y_at_radec_0 = util.map_coord2pix(-self._ra_at_xy_0, -self._dec_at_xy_0, 0, 0,
                                                                      self._Ma2pix)
docs: shifts the coordinate system
          :param x_shif: shift in x (or RA)
          :param y_shift: shift in y (or DEC)
          :param pixel_unit: bool, if True, units of pixels in input, otherwise RA/DEC
          :return: updated data class with change in coordinate system