'Calculate mean value of a single time serie in this query.'

def Mean(self):

    if (self.time_series is None):
        raise RuntimeError('Mean    must    be    called    after    Take*().')
    if (not self.time_series):
        return 0
    if (len(self.time_series) != 1):
        raise RuntimeError('Can    only    return    mean    for    a    single    time    serie.')
    return self.time_series[0].Mean()
