[vtkusers] Source derived from vtkAlgorithm

[Jeff Baumes]

> I spent quite a while paring some of those down and combining them trying to
> get the desired functionality. However, I'm still having some issues. The
> pipeline code gets pretty complex in a big hurry.

You may want to look at this special-purpose data object. Note that
your new type must inherit from vtkDataObject or subclass.

Rendering/vtkLabelHierarchy (new data object subclass)
Rendering/vtkLabelHierarchyAlgorithm (new superclass for algorithms)
Rendering/vtkPointSetToLabelHierarchy (algorithm that produces
vtkLabelHierarchy)

Jeff