BUG: Was reporting total difference, not mean.  Corrected to report mean difference for those pixels that are different.
BUG: Was reporting total difference, not mean.  Corrected to report mean difference for those pixels that are different.

KitwareMedical/ITKTubeTK,thewtex/TubeTK,LucasGandel/TubeTK,cdeepakroy/TubeTK,KitwareMedical/ITKTubeTK,aylward/ITKTubeTK,sumedhasingla/TubeTK,KitwareMedical/TubeTK,sumedhasingla/TubeTK,cdeepakroy/TubeTK,LucasGandel/TubeTK,cdeepakroy/TubeTK,LucasGandel/TubeTK,KitwareMedical/TubeTK,cdeepakroy/TubeTK,thewtex/TubeTK,KitwareMedical/ITKTubeTK,matthieuheitz/TubeTK,sumedhasingla/TubeTK,aylward/ITKTubeTK,aylward/ITKTubeTK,matthieuheitz/TubeTK,KitwareMedical/TubeTK,aylward/ITKTubeTK,thewtex/TubeTK,matthieuheitz/TubeTK,thewtex/TubeTK,LucasGandel/TubeTK,KitwareMedical/ITKTubeTK,sumedhasingla/TubeTK,matthieuheitz/TubeTK,KitwareMedical/TubeTK