Hide the display nodes during cropping

This avoids auto window level calculation on the temporary volumes
(cropped volumes) used only during the roi registration.

It would be better to avoid using other Slicer modules during the landmark
refinement calculation to avoid unnecessary and temporary scene modifications.

BRAINSfit just on the local ROIs can run at about 4 Hz.  Using other Slicer modules
in landmark refinement brings the overall refresh rate to 1 Hz.

Talking more directly to BRAINSfit (or even to a special registration algorithm)
would yield better performance.