Read in the resolution matrix of an inversion

        Parameters
        ----------
        tomodir: string
            directory path to a tomodir

def _read_resm_m(self, tomodir):
        """Read in the resolution matrix of an inversion

        Parameters
        ----------
        tomodir: string
            directory path to a tomodir

        """
        resm_file = tomodir + os.sep + 'inv' + os.sep + 'res_m.diag'
        if not os.path.isfile(resm_file):
            print('res_m.diag not found: {0}'.format(resm_file))
            print(os.getcwd())
            return 1

        # read header
        with open(resm_file, 'rb') as fid:
            first_line = fid.readline().strip()
            header_raw = np.fromstring(first_line, count=4, sep=' ')
            header_raw
            # nr_cells = int(header_raw[0])
            # lam = float(header_raw[1])

            subdata = np.genfromtxt(fid)
            print(subdata.shape)
            pid = self.parman.add_data(subdata[:, 0])
            self.assignments['resm'] = pid