Compute a model response, i.e. complex impedances

        Parameters
        ----------
        log_sigma : 1xN or 2xN numpy.ndarray
            Model parameters log sigma, N the number of cells. If first
            dimension is of length one, assume phase values to be zero

        Returns
        -------
        measurements : Nx2 numpy nd array
            Return log_e sigma values of computed forward response

def forward_complex(self, log_sigma):
        """Compute a model response, i.e. complex impedances

        Parameters
        ----------
        log_sigma : 1xN or 2xN numpy.ndarray
            Model parameters log sigma, N the number of cells. If first
            dimension is of length one, assume phase values to be zero

        Returns
        -------
        measurements : Nx2 numpy nd array
            Return log_e sigma values of computed forward response
        """
        m = 1.0 / np.exp(log_sigma)
        tdm = self._get_tdm(m)
        measurements = tdm.measurements()
        # import IPython
        # IPython.embed()
        # convert R to log sigma
        measurements[:, 0] = np.log(1.0 / measurements[:, 0])
        return measurements