ENH: The top transform is the only one active for registration.

After a new transform is added only that new one will be active for
registration.

Change-Id: I3bbc9fde926bbd687ea5b3920844bcd4bb88ca0c
Ensure only top transform is active

When reading a transform from file or importing from ITK set only the
most recent transform to be active for registration.

Change-Id: I14465c0f828a1d0bdc01ba572d009804d1fb386e
Merge topic 'SIMPLEITK-522_TransformOptimizationMostRecent' into next

f86a2ef2 Ensure only top transform is active
1d2e8208 ENH: The top transform is the only one active for registration.