Save computation time in DDP and FDDP solvers + fixed bug in DDP

This is a list of changes in this PR:

1. Stored the V_{xx} at the rollout state (not the guess one) in the SolverFDDP.
2. Changed the gap definition in SolverFDDP to make it compatible with PDF.
3. Allocated onces the following data: xs, us, xs_try, us_try and gaps. This was done for all solvers.
4. Created a computeGains function inside SolverFDDP which allows us to use SolverBoxDDP with SolverFDDP.
5. Fixed a bug in the SolverDDP, wrong computation of Q_x and Q_u. With this, I have to change few log files (see 63155a6a for more details).
6. Little improvement in the documentation of SolverDDP and SolverFDDP.