docs: def save_to_rho_file(self, filename, cid_mag, cid_pha=None):
          """Save one or two parameter sets in the rho.dat forward model format
  
          Parameters
          ----------
          filename: string (file path)
              output filename
          cid_mag: int
              ID of magnitude parameter set
          cid_pha: int, optional
              ID of phase parameter set. If not set, will be set to zeros.
  
          """
          mag_data = self.parsets[cid_mag]
          if cid_pha is None:
              pha_data = np.zeros(mag_data.shape)
          else:
              pha_data = self.parsets[cid_pha]
  
          with open(filename, 'wb') as fid:
              fid.write(
                  bytes(
                      '{0}\n'.format(self.grid.nr_of_elements),
                      'utf-8',
                  )
              )
              np.savetxt(
                  fid,
                  np.vstack((
                      mag_data,
                      pha_data,
                  )).T,
                  fmt='%f %f'
              )