Enable hardware selection for vtkCellGrid

• 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.
• The new TestCellGridRenderedAreaPickerWithPolyData.py extends the selection to include vtkPolyData in the same renderer used by vtkCellGrid.