COMP: Fixes signed/unsigned comparison warnings in RidgExtractor class
COMP: Fixes signed/unsigned comparison warnings in RidgExtractor class

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