Get GO IDs in GO slims.

def _init_goslims(self, dagslim):
        """Get GO IDs in GO slims."""
        go2obj_main = self.gosubdag.go2obj
        go2obj_slim = {go for go, o in dagslim.items() if go in go2obj_main}
        if self.gosubdag.relationships:
            return self._get_goslimids_norel(go2obj_slim)
        return set(dagslim.keys())