WIP: Use exact equation for Vxx if regularisation is used

This continues from an email discussion with @nmansard and @cmastall and adds the exact formula for Vxx if regularisation is used. At present, the exact formula for Vx is used when control regularisation is used, while the simplified formula is used when no regularisation is applied. However, for Vxx only the simplified equation is implemented.

Two notes:

1. This passes unit tests for me in Python. Vxx may not be covered in Python(?)
2. The convergence in C++ did not show any difference between using the short and the exact formula (tested with quadrupedal_gaits.py and the walk script with unlimited iteration limit)
3. In Python, the test_contacts unit test fails for me on devel

@nmansard and @cmastall, can you please have a look whether this corresponds to our email conversation or whether I have missed something?

Many thanks, Wolf