[vtkusers] How to improve the speed

[John Biddiscombe]

> How to improve the processing speed when clipping 3D dataset(vtk4.0)

This is somethig I've been wanting to do for some time. 
1) Add bounding boxes to those implicit functions that are bounded (or even semi-bounded in the case of a plane etc)
2) Add FindCellsWithinBBox to locators (I've done this with some of my own locators, but not to the vtkCellLocator class)
3) Modifiy ClipPolyData (+ClipDataSet) so that it uses the above

that'd help in many cases as the locator/BBox test could be O(log(N)) or similar as opposed to testing each cell which is currently done and is O(N).

If anyone whats to suggest a better approach...

JB