title: how to write multiple vtkUnstructuredGrid in one .vtu file
body: <p>I want to write multiple unstructured grids in one .vtu file.</p>
  
  <p>I tried below. MakeHexagonalPrism() and MakeHexahedron() return vtkSmartPointer type. 
  The result is there was only one unstructured grid in the output file.</p>
  
  <pre><code>  vtkSmartPointer&lt;vtkXMLUnstructuredGridWriter&gt; writer =
      vtkSmartPointer&lt;vtkXMLUnstructuredGridWriter&gt;::New();
    writer-&gt;SetFileName(filename.c_str());
    writer-&gt;SetInputData(MakeHexagonalPrism());
    writer-&gt;SetInputData(MakeHexahedron());
    writer-&gt;Write();
  </code></pre>
  
  <p>I also tried below. The type of cellArray1 and cellArray2  is vtkSmartPointer. The result is there was only one type of unstructured grid in the output file.</p>
  
  <pre><code>  vtkSmartPointer&lt;vtkUnstructuredGrid&gt; unstructuredGrid =
      vtkSmartPointer&lt;vtkUnstructuredGrid&gt;::New();
    unstructuredGrid-&gt;SetPoints(points);
    unstructuredGrid-&gt;SetCells(VTK_TETRA, cellArray1);
    unstructuredGrid-&gt;SetCells(VTK_WEDGE, cellArray2);
  </code></pre>
  
  <p>I do not know how to write multiple unstructured grids in one .vtu file.
  I'd be grateful for any hints.</p>