Return a newly added |_Relationship| instance.

def add_relationship(self, reltype, target, rId, is_external=False):
        """
        Return a newly added |_Relationship| instance.
        """
        rel = _Relationship(rId, reltype, target, self._baseURI, is_external)
        self[rId] = rel
        if not is_external:
            self._target_parts_by_rId[rId] = target
        return rel