ENH:  Permitting access to shrinking per dimension.

In the ImageRegistrationMethod class, the image is
subsampled using the itkShrinkImageFilter.  Currently
the only option is to shrink each dimension by a
constant factor which is problematic for extremely
anisotropic data.  This change opens up to the user
the possibility of specifying the shrink factor per
dimension per level.

Change-Id: I2f5af406a82be8cf97c2011534bc2aa8e289929e
Merge topic 'ShrinkPerDimension'

e432983 ENH:  Permitting access to shrinking per dimension.