Added first take on the frame sorter class

This should potentially go into DCMTK later
Use SpacingBetweenSlices instead of SliceThickness

See http://dclunie.blogspot.com/2013/10/how-thick-am-i-sad-story-of-lonely-slice.html for details.

Need to confirm with David how legit this is, since SpacingBetweenSlices is not
included in segmentation image IOD.