Group GO IDs by namespace.

def get_ns2goids(self, goids):
        """Group GO IDs by namespace."""
        ns2goids = cx.defaultdict(set)
        go2nt = self.go2nt
        for goid in goids:
            ns2goids[go2nt[goid].NS].add(goid)
        return {ns:gos for ns, gos in ns2goids.items()}