BUG: Fix 0002466, MRMLIDImageIO crashes with DWMRI volume

http://www.na-mic.org/Bug/view.php?id=2466

Restore handling of b-values and gradients, and make code consistent
with DWIConvert: use the sqrt(b/b_max) scaling factor as documented
on the NA-MIC wiki page:

http://wiki.na-mic.org/Wiki/index.php/NAMIC_Wiki:DTI:Nrrd_format

Reviewed-by: Francois Budin <fbudin@unc.edu>
Reviewed-by: Lauren O'Donnell <odonnell@bwh.harvard.edu>
Reviewed-by: Steve Pieper <pieper@isomics.com>
Tested-by: Isaiah Norton <inorton@bwh.harvard.edu>

From: Isaiah Norton <inorton@bwh.harvard.edu>

git-svn-id: http://svn.slicer.org/Slicer4/trunk@24724 3bd1e089-480b-0410-8dfb-8563597acbee