Fixed vtkXdmfReader for parallel structured data.

Readers are no longer passed a sub-extent if they say
they support piece requests. Added extent splitting logic.

Change-Id: If245a347cc71b300f9deca6bf66a559f922cc96d
Merge topic 'fixed-xdmf-reader' into master

06685ac2 Fixed vtkXdmfReader for parallel structured data.