Add SetIdentity method to transform interface

Most of the itk transforms have this method available. However, the
composite transform has not straight forward concept what this should
be, so it throws an exception.

Change-Id: Ib5acdfaab8d8c613172d7867c13e22f8502d08bf
Adding comprehensive testing for SetIdentity.

Change-Id: I3a58a3bf465387ad593936efca915df617eedbb9
Merge topic 'SIMPLEITK-573_AddAdditionalTransformMethods' into next

eb07cb81 Adding comprehensive testing for SetIdentity.
bb45c766 Add SetIdentity method to transform interface