BUG: fix DWI loading in DICOM module by using DWIConvert module

Adapt to updates in DWIConvert by creating a target DWI volume
to store the output.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@21862 3bd1e089-480b-0410-8dfb-8563597acbee