Adds a term to the term layer
        @type term_obj: L{Cterm}
        @param term_obj: the term object

def add_term(self,term_obj):
        """
        Adds a term to the term layer
        @type term_obj: L{Cterm}
        @param term_obj: the term object
        """
        if self.term_layer is None:
            self.term_layer = Cterms(type=self.type)
            self.root.append(self.term_layer.get_node())
        self.term_layer.add_term(term_obj)