[vtkusers] Polyhedron support in Xdmf3

[Armin Wehrfritz]

Hei!

The XDMF file format has been missing support for polyhedral cells, as
it was pointed out in several places/discussions, e.g. [1-4].

Since the implementation seemed straight forward, I gave it a try and
implemented it to the Xdmf3 reader/writer in VTK. I created a merge
request in the hope to get comments and the code eventually merged into VTK:
https://gitlab.kitware.com/vtk/vtk/merge_requests/1255

I found two small bugs along the way and you can find a fix for them in
this merge request:
https://gitlab.kitware.com/vtk/vtk/merge_requests/1251

I implemented this directly in VTK since the upstream Xdmf repository is
already out of date and needs to be update (see [5] for more details).

Best regards,
Armin



[1] http://www.kitware.com/pipermail/xdmf/2015-December/000895.html
[2] http://www.xdmf.org/index.php/V3_Road_Feature_Request
[3] http://public.kitware.com/Bug/view.php?id=14528
[4] http://visitbugs.ornl.gov/issues/1279
[5] http://www.kitware.com/pipermail/xdmf/2015-December/000893.html