Ensure that a ctable exists for ``sid``, then return it.

def _ensure_ctable(self, sid):
        """Ensure that a ctable exists for ``sid``, then return it."""
        sidpath = self.sidpath(sid)
        if not os.path.exists(sidpath):
            return self._init_ctable(sidpath)
        return bcolz.ctable(rootdir=sidpath, mode='a')