code cleanup (threshold and contour computation)
update GUI StartSekeleton (fix slider step, display of buttons)
create HypoGrowthApp class, and use it in StartSkeleton.m
use HypoGrowthApp class for ValidateThres.m
use HypoGrowthApp in ValidateContour.m
use HypoGrowthApp for ValidateSkeleton.m dialog
use HypoGrowthApp in StartElongation dialog
update HypoGrowthApp data structure, and update StartElongation