Load one parameter set from a rho*.mag or rho*.pha file produced by
        CRTomo.

        Parameters
        ----------
        filename : string, file path
            Filename to loaded data from
        columns : int or iterable of ints, optional
            column(s) to add to the manager. Defaults to 2 (third column).

        Returns
        -------
        pid : int or list of ints
            ID(s) of parameter set

def load_inv_result(self, filename, columns=2):
        """Load one parameter set from a rho*.mag or rho*.pha file produced by
        CRTomo.

        Parameters
        ----------
        filename : string, file path
            Filename to loaded data from
        columns : int or iterable of ints, optional
            column(s) to add to the manager. Defaults to 2 (third column).

        Returns
        -------
        pid : int or list of ints
            ID(s) of parameter set

        """
        assert os.path.isfile(filename)
        try:
            iterator = iter(columns)
        except TypeError:
            # not iterable
            iterator = [columns, ]

        pid_list = []
        for column in iterator:
            data = np.loadtxt(filename, skiprows=1)
            pid = self.add_data(data[:, column])
            pid_list.append(pid)

        if len(pid_list) == 1:
            return pid_list[0]
        else:
            return pid_list