In conversion of vtkRectilinearGrid to XdmfRectilinearGrid,
vtkRectilinearGrid::XCoordinates should be set to XCoords of
XdmfRectilinearGrid and vtkRectilinearGrid::ZCoordinates should be
set to ZCoords of XdmfRectilinearGrid respectively.

Both XdmfRectilinearGrid and vtkRectilinearGrid use the topology that
X (or I) indices varies at first.