numpy version check

[akreshuk]

in hessian_eigenvectors.py, the check is made like:
assert numpy.__version__ >= '1.8.0'
Obviously, it would fail now that we have 1.11.1. This issue is trivial to fix, but we should check for such asserts all over.

[stuarteberg]

Ah, good point. BTW: I don't think we're actually using hessian_eigenvectors() anywhere. (I thought I would use it in the IIBoost workflow, but I didn't need it in the end.)

Are you using that function? If so, we should consider creating an ND python API for vigra::linalg::symmetricEigensystem(), which might be faster. In the mean time, you can try iiboost.computeEigenVectorsOfHessianImage(), which is already exposed to Python (see opIIBoostFeatureSelection.py for example usage.)