Created solver abstraction and implemented for the SolverDDP, SolverFDDP and SolverKKT

This PR makes the following changes:

• Created a solver abstraction (i.e. SolverAbstract class) inside solver.py
• Integrated this class into SolverDDP, SolverFDDP and SolverKKT classes.
• Implemented setCandidate, models and datas inside SolverAbstract
• Removed legacy variable b inside SolverDDP.forwardPass method. and it tackles issue #79 (closed).

@nmansard do you have any further suggestions? I believe that we could do more things but it might be premature in this phase.