Add homogeneous models to one or all tomodirs. Register those as
        forward models

        Parameters
        ----------
        magnitude : float
            Value of homogeneous magnitude model
        phase : float, optional
            Value of homogeneous phase model. Default 0
        frequency : float, optional
            Frequency of of the tomodir to use. If None, then apply to all
            tomodirs. Default is None.

def add_homogeneous_model(self, magnitude, phase=0, frequency=None):
        """Add homogeneous models to one or all tomodirs. Register those as
        forward models

        Parameters
        ----------
        magnitude : float
            Value of homogeneous magnitude model
        phase : float, optional
            Value of homogeneous phase model. Default 0
        frequency : float, optional
            Frequency of of the tomodir to use. If None, then apply to all
            tomodirs. Default is None.
        """
        if frequency is None:
            frequencies = self.frequencies
        else:
            assert isinstance(frequency, Number)
            frequencies = [frequency, ]

        for freq in frequencies:
            pidm, pidp = self.tds[freq].add_homogeneous_model(magnitude, phase)
            self.a['forward_rmag'][freq] = pidm
            self.a['forward_rpha'][freq] = pidp