Interact with segy in gather mode

        Returns
        -------
        gather : Gather

def gather(self):
        """
        Interact with segy in gather mode

        Returns
        -------
        gather : Gather
        """
        if self.unstructured:
            raise ValueError(self._unstructured_errmsg)

        if self._gather is not None:
            return self._gather

        self._gather = Gather(self.trace, self.iline, self.xline, self.offsets)
        return self._gather