code: def add_channel_cb(self, viewer, channel):
          
          fitsimage = channel.fitsimage
          fitssettings = fitsimage.get_settings()
          for name in ['cuts']:
              fitssettings.get_setting(name).add_callback(
                  'set', self.cutset_cb, fitsimage)
          fitsimage.add_callback('transform', self.transform_cb)
  
          rgbmap = fitsimage.get_rgbmap()
          rgbmap.add_callback('changed', self.rgbmap_cb, fitsimage)
  
          # add old highlight set to channel external data
          channel.extdata.setdefault('thumbs_old_highlight', set([]))
docs: Called when a channel is added from the main interface.
          Parameter is channel (a bunch).