- This commit adds two new properties on `vtkSelectionNode` to indicate
  the cell type index that was picked in a `vtkCellGrid` and whether a cell
  or side was picked.
- The `vtkDGRenderResponder` and it's OpenGL shaders encode various
  indices during the new picking passes into the framebuffer.
- The hardware selector decodes the pixel information encoded by
  `vtkDGRenderResponder` in the framebuffer into `vtkSelectionNode` objects.
- The `vtkAreaPicker`, `vtkRenderedAreaPicker` and `vtkHardwarePicker`
  can now determine whether a `vtkDataObject` was picked.
  Earlier, it only recognized derived instances of `vtkDataSet`.
  This is required because `vtkCellGrid` directly inherits `vtkDataObject`.
- The new `TestCellGridHardwarePicker.py` unit test verifies basic
  point and click selection using `vtkHardwarePicker`. It tests that
  the picker populates correct information in the different pick
  result indices.
- The new `TestCellGridRenderedAreaPicker.py` unit test verifies rubberband
  selection using `vtkRenderedAreaPicker`. It tests that the picker
  populates correct values for the different information integer keys
  on selection nodes.