[vtkusers] Using an octree

[David Thompson]

> If I include
> #include <octree/octree.h>
> 
> I get a whole bunch of these errors.
You need to include vtkObjectBase.h before octree/octree.h so that
vtkstd will be defined (or #define vtkstd std).

	David

> 
> /home/doriad/src/ParaView3/VTK/Utilities/octree/octree.h:9: error:
> expected type-specifier before 'vtkstd'
> /home/doriad/src/ParaView3/VTK/Utilities/octree/octree.h:9: error:
> expected '>' before 'vtkstd'
> /home/doriad/src/ParaView3/VTK/Utilities/octree/octree.h:23: error:
> ISO C++ forbids declaration of 'octree_node' with no type
> /home/doriad/src/ParaView3/VTK/Utilities/octree/octree.h:23: error:
> expected ';' before '<' token
> 
> Any reason why I can't use the octree class?
>