BUG: gracefully handle case where SimpleITK is not available

On continuous builds and local developer builds SimpleITK is not
enabled by default, so we need to test for it and avoid raising
an error that prevents the module from loading.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22120 3bd1e089-480b-0410-8dfb-8563597acbee