Fixed a bug in reading vtu data with polyhedron cells

Paraview bug: http://www.paraview.org/Bug/view.php?id=13892
If there are polydhedron cells in multiple pieces of a vtu file, the
vtkXMLUnstructuredDataReader will fail to read properly the polyhedron
cell and faces info, except the cell in first piece. The bug is in the
logic of figuring out the offset of faces and points when there are
multiple pieces containing polyhedron cells.

Change-Id: Ic1100bc816c1e0f51cc45bc34d26ec92799bd36f
Added a test for XMLUnstructuredGridReader

Change-Id: I8ebd896d483684cac7df4d7313f4fa8541437ab3
Merge topic 'polyhedron_pieces_in_vtu' into master

cd39d68b Added a test for XMLUnstructuredGridReader
8bc56514 Fixed a bug in reading vtu data with polyhedron cells