docs: def xd(self):
          """get xarray dataset file handle to LSM files"""
          if self._xd is None:
              path_to_lsm_files = path.join(self.lsm_input_folder_path,
                                            self.lsm_search_card)
  
              self._xd = pa.open_mfdataset(path_to_lsm_files,
                                           lat_var=self.lsm_lat_var,
                                           lon_var=self.lsm_lon_var,
                                           time_var=self.lsm_time_var,
                                           lat_dim=self.lsm_lat_dim,
                                           lon_dim=self.lsm_lon_dim,
                                           time_dim=self.lsm_time_dim,
                                           loader=self.pangaea_loader)
  
              self.lsm_time_dim = 'time'
              self.lsm_time_var = 'time'
          return self._xd