Initial vtkCellGrid support in ParaView.
- Make vtkPVDataInformation and vtkPVArrayInformation support cell-grids and their cell-attributes. The latter are all presented as cell-centered arrays in ParaView, for lack of a better alternative.
- Make the glyph-3d representation check its mapper. This avoids a crash when users attempt to force the glyph representation on a composite cell-grid representation.
- Make a "CellGrid" representation. Lots of copy-pasta to get a cell-grid representation. It has "Outline" and "Surface" styles.