get xarray dataset file handle to LSM files

def xd(self):
        """get xarray dataset file handle to LSM files"""
        if self._xd is None:
            # download files if the user requests
            if None not in (self.download_start_datetime, self.download_end_datetime):
                self._download()

            self._xd = super(ERAtoGSSHA, self).xd
            self._xd.lsm.lon_to_180 = True
        return self._xd