Add Support for Polyhedron Cells

[Taylor McDonnell]

This package doesn't appear to support VTKCellTypes.VTK_POLYHEDRON cells.

The following is an example VTK file for this cell type. As far as I can tell, there doesn't seem to be a way to add the faces and faceoffsets fields in WriteVTK.

<?xml version="1.0"?>
<VTKFile type="UnstructuredGrid" version="0.1" byte_order="LittleEndian" header_type="UInt32" compressor="vtkZLibDataCompressor">
  <UnstructuredGrid>
    <Piece NumberOfPoints="8" NumberOfCells="1">
      <PointData>
      </PointData>
      <CellData>
      </CellData>
      <Points>
        <DataArray type="Float32" Name="Points" NumberOfComponents="3" format="ascii" RangeMin="1.7320508075688772" RangeMax="1.7320508075688772">
          -1 -1 -1 1 -1 -1
          1 1 -1 -1 1 -1
          -1 -1 1 1 -1 1
          1 1 1 -1 1 1
        </DataArray>
      </Points>
      <Cells>
        <DataArray type="Int64" Name="connectivity" format="ascii" RangeMin="0" RangeMax="7">
          0 1 2 3 4 5
          6 7
        </DataArray>
        <DataArray type="Int64" Name="offsets" format="ascii" RangeMin="8" RangeMax="8">
          8
        </DataArray>
        <DataArray type="UInt8" Name="types" format="ascii" RangeMin="42" RangeMax="42">
          42
        </DataArray>
        <DataArray type="Int64" Name="faces" format="ascii" RangeMin="0" RangeMax="7">
          6 4 0 3 2 1
          4 0 4 7 3 4
          4 5 6 7 4 5
          1 2 6 4 0 1
          5 4 4 2 3 7
          6
        </DataArray>
        <DataArray type="Int64" Name="faceoffsets" format="ascii" RangeMin="31" RangeMax="31">
          31
        </DataArray>
      </Cells>
    </Piece>
  </UnstructuredGrid>
</VTKFile>

[Juan Ignacio Polanco]

That shouldn't be too difficult to add.

However, I'm not really familiar with the specification of polyhedron cells. Could you provide some information on the definition of the faces and faceoffsets fields?