docs: def write_crmod_volt(self, filename, mid):
          """Write the measurements to the output file in the volt.dat file
          format that can be read by CRTomo.
  
          Parameters
          ----------
          filename: string
              output filename
          mid: int or [int, int]
              measurement ids of magnitude and phase measurements. If only one ID
              is given, then the phase column is filled with zeros
  
          """
          ABMN = self._get_crmod_abmn()
  
          if isinstance(mid, (list, tuple)):
              mag_data = self.measurements[mid[0]]
              pha_data = self.measurements[mid[1]]
          else:
              mag_data = self.measurements[mid]
              pha_data = np.zeros(mag_data.shape)
  
          all_data = np.hstack((ABMN, mag_data[:, np.newaxis],
                                pha_data[:, np.newaxis]))
  
          with open(filename, 'wb') as fid:
              fid.write(bytes(
                  '{0}\n'.format(ABMN.shape[0]),
                  'utf-8',
              ))
              np.savetxt(fid, all_data, fmt='%i %i %f %f')