BUG: Use fix image as reference image for bspline transform

Registration transform map from the fixing domain to the moving domain
of the images. Therefore the transform should be initialized to the
fixing image domain.

Change-Id: I7027ac9b375b0b76facb39489496b9b8e0f5e31f
Suggested-by: Ali Ghayoor <ali-ghayoor@uiowa.edu>
Merge topic 'SIMPLEITK-571_AddMoreRegistrationExamples'

ab335ed4 BUG: Use fix image as reference image for bspline transform