Return a go2obj dict for just the user goids.

def get_vals(self, field, goids=None):
        """Return a go2obj dict for just the user goids."""
        go2nt = self.go2nt
        if goids is None:
            goids = set(go2nt)
        return [getattr(go2nt[go], field) for go in goids]