Small fixes and cleanup for vtkCellGrid

1. Some issues with the CellGridSource example program (in Examples/GUISupport/Qt/CellGridSource) have been identified and fixed.
2. The vtkCellGridCellToUnstructuredGrid algorithm now allows empty output in cell-grids without a crash.
3. The cell-grid side filter properly marks itself as modified when its query-parameters are changed.
4. The cell-grid source now produces vtkDGVert cells with the proper constant function space for its shape and vector cell-attributes.
5. The cell-grid sides query has been split into a query and a cache class, the latter of which can be used by other filters.