docs: 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