* Fix typo in a variable name in example code.
* Rename df2dx2 to d2fdx2 - the appropriate naming for the second derivative
* Update CONTRIBUTING.md - add needed python packages, and a virtual-env hint
* Revert "Fix typo in a variable name in example code."
This reverts commit bc10a175345c3e20eab689c47f7d866073a3c233.
* Rename df2dx2 to d2fdx2