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

def load_rho_file(self, filename):
        """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

        """
        pids = self.parman.load_from_rho_file(filename)
        self.register_magnitude_model(pids[0])
        self.register_phase_model(pids[1])
        return pids