BUG: fix loading multivolumes produced by CLIs

This addresses Slicer issue http://www.na-mic.org/Bug/view.php?id=3382.

The patch has been suggested by Jim Miller (see the Slicer issue).

Jim wants to add more comments before this is merged with the Slicer master.

From: Andrey Fedorov <fedorov@bwh.harvard.edu>

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23876 3bd1e089-480b-0410-8dfb-8563597acbee