ENH: ImageRegistration6 is fully converted to ITKv4

Change-Id: Iccc04dfc7303dd826469f39a42409c97dc5a0375
WIP: ImageRegistration7 example is fully converted to ITKv4

It seems that this example is the best place that we can explain about
ScaleEstimators. Work on that later.

Change-Id: Idf6bad24ee0dba573c44a39d20774697ae0b9a85