'Accumulate some connections.'

def accumulate(self, con_idx, csd_xy):

    im_csd = np.imag(csd_xy)
    self._acc[(0, con_idx)] += im_csd
    self._acc[(1, con_idx)] += np.abs(im_csd)
    self._acc[(2, con_idx)] += (im_csd ** 2)
