Convert elastix RecursiveBSplineTransform to ITK BSplineTransform during I/O

[N-Dekker]

Check that elastix RecursiveBSplineTransform data can be converted losslessly to ITK's BSplineTransform format, and then do so, when elastix parameter ITKTransformOutputFileNameExtension is non-empty.

Consider unifying RecursiveBSplineTransform and BSplineTransform for I/O purposes. (Using RecursiveBSplineTransform only internally, for performance reasons.)

[N-Dekker]

TODO Promote the recursiev as default bspline, in Model Zoo, manual, and wiki, Jupiter notebooks.

[N-Dekker]

@ntatsisk Please consider task regarding the Jupiter notebooks.