ENH: Use Transform::Clone() in ImageToImageMetric.

Improves code simplicity and allows the use of CompositeTransform.

Change-Id: I2fa49b51ac622f9b370b72a3bf5dba7424048630
COMP: Avoid MetaIO config conflict with USE_SYSTEM_GDCM.

When a system GDCM with configured with VTK support, its UseGDCM.cmake file
can call include(${VTK_USE_FILE}).  This can cause VTK's include directories
to fall before ITK's, the system can find VTK's metaIOConfig.h first, and
MetaIO build failures will occurue.

Change-Id: Ided8ec692a10fdde423f9f4fec521727d7b4b83e
BUG: itkSingleLevelSetsv4WhitakerImage2DTest output clobber.

Prevent the two tests from writing to the same output file and possibly
clobbering each other.

Change-Id: I721f730ed6663bdf334e4ec674606509379b929a
Merge topic 'itkSingleLevelSetsv4WhitakerImage2DTest'

372dc26 BUG: itkSingleLevelSetsv4WhitakerImage2DTest output clobber.
Merge topic 'ImageToImageClone'

4b11d9d ENH: Use Transform::Clone() in ImageToImageMetric.
Merge topic 'SystemGDCMWithVTK'

8b81130 COMP: Avoid MetaIO config conflict with USE_SYSTEM_GDCM.