get xarray dataset file handle to LSM files

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