code: def change_range_cb(self, setting, value, fitsimage):
          
          if not self.gui_up:
              return
          if fitsimage != self.fv.getfocus_viewer():
              # values have changed in a channel that doesn't have the focus
              return False
          loval, hival = value
          self.colorbar.set_range(loval, hival)
docs: This method is called when the cut level values (lo/hi) have
          changed in a channel.  We adjust them in the ColorBar to match.