Make structure tensor function public and add function to compute hessian matrix
Add missing imports
Add test cases for structure tensor and hessian matrix functions
Add functions to compute eigen values of structure tensor and hessian matrix
Add examples for structure tensor and hessian matrix functions
Add test cases for eigen value functions of structure tensor and hessian matrix
Merge pull request #9 from ahojnnes/orb

Hessian matrix and structure tensor