Fixed BUG #12527. Fixed parallel read issue for unstructured grids.
Fixed BUG #12527. Fixed parallel read issue for unstructured grids.

Fixed the reader to only read unstructure grids on root node when reading in
parallel. For structured grids, the update extents are respected. While for
composite datasets, we distribute the leaves among processes.

cjh1/Xdmf2,cjh1/Xdmf2,cjh1/Xdmf2