Apply only t0s

def apply_t0(self, hits):
        """Apply only t0s"""
        if HAVE_NUMBA:
            apply_t0_nb(
                hits.time, hits.dom_id, hits.channel_id, self._lookup_tables
            )
        else:
            n = len(hits)
            cal = np.empty(n)
            lookup = self._calib_by_dom_and_channel
            for i in range(n):
                calib = lookup[hits['dom_id'][i]][hits['channel_id'][i]]
                cal[i] = calib[6]
            hits.time += cal
        return hits