BUG: Fix ResampleImage XML

The output volume had a string type where it really should be an image to
be correctly integrated with slicer's GUI.
The optionnal image didn't have any channel which prevented it from
working.

Change-Id: I3cca295456dd16d268185e5004cf4ad6a1937f69
Merge topic 'FixResampleXML'

af3fc0a BUG: Fix ResampleImage XML