Get the top GO for the set of goids_all.

def _get_goroot(self, goids_all, namespace):
        """Get the top GO for the set of goids_all."""
        root_goid = self.consts.NAMESPACE2GO[namespace]
        if root_goid in goids_all:
            return root_goid
        root_goids = set()
        for goid in goids_all:
            goterm = self.gosubdag.go2obj[goid]
            if goterm.depth == 0:
                root_goids.add(goterm.id)
        if len(root_goids) == 1:
            return next(iter(root_goids))
        raise RuntimeError("UNEXPECTED NUMBER OF ROOTS: {R}".format(R=root_goids))