Displacement field to (BSpline) Transform

Hi @Mikhail ,

To obtain a displacement field from a b-spline or other transform, use TransformToDisplacementFieldFilter.

This filter is available in the itk-5.3rc1 Python packages.

Try

displacement_field = itk.transform_to_displacement_field_filter(bspline_transform, 
                                                                                                     use_reference_image=True, 
                                                                                                     reference_image=my_reference_image)