BUG: Add GetInverse for IdentityTransform

This function is trivially implemented as an identity transform is
it's own inverse.

Change-Id: I301ca1e2b2584c0ab77c10b727eaccd4ca68a954
ENH: Adding SetIdentity method to base DisplacementField class

While missing from the Transform base class, most transform implement
a SetIdentity method. This method is useful to present a uniform
simple interface to ITK's transform class.

It set the displacement and the inverse displacement fields, if
provided, to zero vectors.

Change-Id: I2881a1c77456c985d1d4c01c10678c6ffa1fc47f
Merge topic 'AddDisplacementFieldSetIdentity'

5df4fa99 ENH: Adding SetIdentity method to base DisplacementField class
Merge topic 'AddIdentityTransformGetInverse'

7cd6c6e0 BUG: Add GetInverse for IdentityTransform