'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):

    ap = 0.0
    for t in np.arange(0.0, 1.1, 0.1):
        if (np.sum((rec >= t)) == 0):
            p = 0
        else:
            p = np.max(prec[(rec >= t)])
        ap += (p / 11.0)
    return ap
