* Add ComputeCellsBounds / GetCellsBounds methods. Cells bounds ignore points
   that do not belong to any cell, as vtkPolyData::ComputeBounds() use to do.
 * Use ComputeBounds / GetBounds from vtkPointSet for consistency with
   other vtkPointSet subclasses.
 * Mappers use CellsBounds for retrocompatibility

 See Documentation/release/dev/PolyDataCellsBounds.md for more details