Load a CRMod measurement file (commonly called volt.dat)

        Parameters
        ----------
        filename: string
            path to filename

        Returns
        -------
        list
            list of measurement ids

def load_crmod_volt(self, filename):
        """Load a CRMod measurement file (commonly called volt.dat)

        Parameters
        ----------
        filename: string
            path to filename

        Returns
        -------
        list
            list of measurement ids
        """
        with open(filename, 'r') as fid:
            nr_of_configs = int(fid.readline().strip())
            measurements = np.loadtxt(fid)
            if nr_of_configs != measurements.shape[0]:
                raise Exception(
                    'indicated number of measurements does not equal ' +
                    'to actual number of measurements')
        ABMN = self._crmod_to_abmn(measurements[:, 0:2])
        if self.configs is None:
            self.configs = ABMN
        else:
            # check that configs match
            if not np.all(ABMN == self.configs):
                raise Exception(
                    'previously stored configurations do not match new ' +
                    'configurations')

        # add data
        cid_mag = self.add_measurements(measurements[:, 2])
        cid_pha = self.add_measurements(measurements[:, 3])
        return [cid_mag, cid_pha]