def bind(self, isnap, istep):
self._isteps[isnap] = istep<EOL>self.sdat.steps[istep].isnap = isnap<EOL>
Register the isnap / istep correspondence.

        Users of :class:`StagyyData` should not use this method.

        Args:
            isnap (int): snapshot index.
            istep (int): time step index.