[vtkusers] How do you turn on/off a pipeline?

[John Biddiscombe]

>A simple question:
>	How does one turn on or off a particular VTK pipeline?  Or how 
>does one disconnect a pipeline??

Write a simple filter which takes an input, and generates an output, all it
does is copystructure from input to output and pass point and cell data,
override the update mechanism so that when flag=false, output doesn't get
updated, when flag=true it acts as usual.

We have one here. If it is in good shape (I haven't used it lately) I'll
add it to the nightlies - though it may not conform to the correct 'style'.
If not, it isn't hard to make one, though I suspect ours doen't correctly
handle all intricate details like streaming properly.

John B