[vtkusers] Cache downsampled image set

[TJ Wilkinson]

I have an image set that is 1.3GB.  I would like to downsample it into a
new volume and cache that volume in memory.  I have been successful in all
this except for loading the entire downsampled volume into memory.  Even
with vtkImageStreamer (whose documentation is a bit sparse), when I call
update() to hopefully load the DOWNSAMPLED volume into memory, it must try
and load the entire set at some point.

If anyone has any experience with large data sets, streaming, or working
around a problem like that I described, please let me know what your
solution is.  

Sincerely,

Todd