[vtkusers] PolyDataExtractNormals example doers not work as expected with cell normals

[Alex Malyushytskyy]

I needed to extract cell normals generated by vtkPolyDataNormals
and tried to use code from
http://www.vtk.org/Wiki/VTK/Examples/Cxx/PolyData/PolyDataExtractNormals
on two versions of vtk (5.4.2 and  5.6.1)

On both cell normals were not working as expected from example.

First of all vtkPolyDataNormals generates vtkFloatArray instead of
vtkDoubleArray as an example,
second name of cell array is "Normals" and not "cellNormals"

I suggest to change example

from:
 ///////////////////////////////////////////////////////////////////////////////////
  // Double cell normals
  vtkSmartPointer<vtkDoubleArray> normalData4 =
    vtkDoubleArray::SafeDownCast(polydata->GetCellData()->GetNormals("cellNormals"));

  if(normalData4)
    {
    std::cout << "There are " << normalData4->GetNumberOfComponents()
              << " components in NormalData4 (double Cell \"Cell
Normals\")" << std::endl;
    return true;
    }


to:
 ///////////////////////////////////////////////////////////////////////////////////
  // Float cell normals

  vtkSmartPointer<vtkFloatArray > normalData4 =
    vtkDoubleArray::SafeDownCast(polydata->GetCellData()->GetNormals("Normals"));

  if(normalData4)
    {
    std::cout << "There are " << normalData4->GetNumberOfComponents()
              << " components in NormalData4 (float Cell \"Cell
Normals\")" << std::endl;
    return true;
    }


Regards,
  Alex