creates a SQLAlchemy Table object with the appropriate number of
        columns given the number of feeds

def _datatable_factory(self):
        """
        creates a SQLAlchemy Table object with the appropriate number of
        columns given the number of feeds
        """
        feed_cols = ['feed{0:03d}'.format(i + 1) for i in range(self.n_feeds)]
        feed_cols = ['override_feed000'] + feed_cols + ['failsafe_feed999']

        ind_sqlatyp = indexingtypes[self.index.indimp].sqlatyp
        dat_sqlatyp = datadefs[self.dtype.datadef].sqlatyp

        atbl = Table(self.name, Base.metadata,
                     Column('indx', ind_sqlatyp, primary_key=True),
                     Column('final', dat_sqlatyp),
                     *(Column(fed_col, dat_sqlatyp) for fed_col in feed_cols),
                     extend_existing=True)
        
        self.dt_feed_cols = feed_cols[:]
        self.dt_all_cols = ['indx', 'final'] + feed_cols[:]
        return atbl