additional checks for displacement transform test

Change-Id: I959f95b43b68e9cc8b23980a6f794d229ad612ea
DOC: adding that the input displacement image is transferred

Change-Id: I4d5892b7052c3cf44e63832e1e4965ded76998d1
Merge topic 'SIMPLEITK-537_AddDisplacementFieldSupport' into next

68e001bc DOC: adding that the input displacement image is transferred
2221db50 additional checks for displacement transform test