Return modeled measurements

        1. dimension: frequency
        2. dimension: config-number
        3. dimension: 2: magnitude and phase (resistivity)

def measurements(self):
        """Return modeled measurements

        1. dimension: frequency
        2. dimension: config-number
        3. dimension: 2: magnitude and phase (resistivity)

        """
        m_all = np.array([self.tds[key].measurements() for key in
                          sorted(self.tds.keys())])
        return m_all