use vtkCellArray in vtkPolyhedron and harness the new unstructured layout

- SetCellFaces method helps to define the polyhedron
- GetCellFaces provides a vtkCellArray that could be used to add the cell to a vtkUnstructuredGrid
- GlobalFaces now uses vtkCellArray
- FaceLocations is not needed anymore and removed
- Faces also uses vtkCellArray to store canonical ids
- LegacyGlobalFaces is a cache to allow backward compatibility at the API level (GetFaces method)
- vtkPolyhedronUtilities is updated accordingly
- vtkPolyhedron is made lighter since this->Polys is now removed and replaced by this->Faces