simplepairwisepotential hessian, improvements to GradienOptimizer cython wrappers, c++ tests

This is a huge pull request, it includes

* simplepairwisepotential hessian: now no issues

* improvements to GradienOptimizer cython wrappers: this only involves ModifiedFire, it should be extended to LBFGS but it's not essential

* c++ tests