calculate average precision, override the default one,
        special 11-point metric

        Params:
        ----------
        rec : numpy.array
            cumulated recall
        prec : numpy.array
            cumulated precision
        Returns:
        ----------
        ap as float

def _average_precision(self, rec, prec):
        """
        calculate average precision, override the default one,
        special 11-point metric

        Params:
        ----------
        rec : numpy.array
            cumulated recall
        prec : numpy.array
            cumulated precision
        Returns:
        ----------
        ap as float
        """
        ap = 0.
        for t in np.arange(0., 1.1, 0.1):
            if np.sum(rec >= t) == 0:
                p = 0
            else:
                p = np.max(prec[rec >= t])
            ap += p / 11.
        return ap