code: def load_rho_file(self, filename):
          
          pids = self.parman.load_from_rho_file(filename)
          self.register_magnitude_model(pids[0])
          self.register_phase_model(pids[1])
          return pids
docs: Load a forward model from a rho.dat file
  
          Parameters
          ----------
          filename: string
              filename to rho.dat file
  
          Returns
          -------
          pid_mag: int
              parameter id for the magnitude model
          pid_pha: int
              parameter id for the phase model