Adds an opinion to the opinion layer
        @type opinion_obj: L{Copinion}
        @param opinion_obj: the opinion object

def add_opinion(self,opinion_obj):
        """
        Adds an opinion to the opinion layer
        @type opinion_obj: L{Copinion}
        @param opinion_obj: the opinion object
        """
        if self.opinion_layer is None:
            self.opinion_layer = Copinions()
            self.root.append(self.opinion_layer.get_node())
        self.opinion_layer.add_opinion(opinion_obj)