[vtkusers] get unstructured grid from vtkXdmf3Reader

[Nico Schlömer]

Hi everyone,

I have an XDMF file which I'm reading with vtk::vtkXdmf3Reader,
```
  vtkXdmf3Reader *xreader = vtkXdmf3Reader::New();
  xreader->SetFileName("data/brick-w-hole.xmf");
  xreader->Update();
```
and I would like to get the corresponding vtk::UnstructuredGrid from
this. For some reason, however,
```
 xreader->GetNumberOfGrids()
```
returns 0. Also, I wouldn't know how to retrieve the grid if there was one.

Is there example code for this around?

Cheers,
Nico