ENH: Made BSplineDeformableTransform (ITKv3) the default write option for bspline transforms

Added a new attribute to vtkMRMLTransformStorageNode: PreferITKv3CompatibleTransforms.
By default PreferITKv3CompatibleTransforms=1 and so bsplines are written as BSplineDeformableTransform (ITKv3).
If PreferITKv3CompatibleTransforms=0 then bsplines are written as BSplineTransform (ITKv4).

When most of the modules support ITKv4 then the default writing format can be changed by setting
PreferITKv3CompatibleTransforms=0 in the vtkMRMLTransformStorageNode constructor.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23591 3bd1e089-480b-0410-8dfb-8563597acbee