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

def add_term(self,term_obj):
        """
        Adds a term object to the layer
        @type term_obj: L{Cterm}
        @param term_obj: the term object
        """
        if term_obj.get_id() in self.idx:
            raise ValueError("Term with id {} already exists!"
                             .format(term_obj.get_id()))
        self.node.append(term_obj.get_node())
        self.idx[term_obj.get_id()] = term_obj