Fixed eigenvalue ordering assertion.

Change-Id: I270519f8bc79abfaaf392dd1c94a29d5e517ea67
FIX: According to VNL docu. EVs are sorted in increasing order.
Added consistent use of assert(...) for eigenvalue ordering.

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

Change-Id: I6878db8f2d6aff7987ece593da4728bd730a046f
Merge topic 'rk-EigFix'

868bd95 Added consistent use of assert(...) for eigenvalue ordering.
8729295 Fixed eigenvalue ordering assertion.