Adds a statement to the attribution layer
        @type statement_obj: L{Cstatement}
        @param statement_obj: the statement object

def add_statement(self,statement_obj):
        """
        Adds a statement to the attribution layer
        @type statement_obj: L{Cstatement}
        @param statement_obj: the statement object
        """
        if self.attribution_layer is None:
            self.attribution_layer = Cattribution()
            self.root.append(self.attribution_layer.get_node())
        self.attribution_layer.add_statement(statement_obj)