Added consistent use of assert(...) for eigenvalue ordering.
Added consistent use of assert(...) for eigenvalue ordering.

ENH: Added assertions after each use of vnl_symmetric_eigensystem(...)
     in itkTubeNJetImageFunction.txx.

Change-Id: I6878db8f2d6aff7987ece593da4728bd730a046f

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