Adds an external reference to the given role identifier in the SRL layer
        @type role_id: string
        @param role_id: the role identifier
        @param external_ref: an external reference object
        @type external_ref: L{CexternalReference}

def add_external_reference_to_role(self,role_id,external_ref):
        """
        Adds an external reference to the given role identifier in the SRL layer
        @type role_id: string
        @param role_id: the role identifier
        @param external_ref: an external reference object
        @type external_ref: L{CexternalReference}
        """
        if self.srl_layer is not None:
            self.srl_layer.add_external_reference_to_role(role_id,external_ref)