Compute the Cumulative Sum at each point 't' of the time series.

def compute_cusum_ts(self, ts):
        """ Compute the Cumulative Sum at each point 't' of the time series. """
        mean = np.mean(ts)
        cusums = np.zeros(len(ts))
        cusum[0] = (ts[0] - mean)
        for i in np.arange(1, len(ts)):
            cusums[i] = cusums[i - 1] + (ts[i] - mean)

        assert(np.isclose(cumsum[-1], 0.0))
        return cusums