Adds a chunk to the chunk layer
        @type chunk_obj: L{Cchunk}
        @param chunk_obj: the chunk object

def add_chunk(self,chunk_obj):
        """
        Adds a chunk to the chunk layer
        @type chunk_obj: L{Cchunk}
        @param chunk_obj: the chunk object
        """
        if self.chunk_layer is None:
            self.chunk_layer = Cchunks(type=self.type)
            self.root.append(self.chunk_layer.get_node())
        self.chunk_layer.add_chunk(chunk_obj)