Warm start classes

[jmirabel]

I have two remarks regarding the warm start classes.

• WarmStartReference.generate calls rnea for each point. We have a performance loss here. We only need to use RNEA algo once for each point regardless of how many times they are used by successive generate calls.
• WarmStartBase.generate should provide as a default behavior a warm start based on the previous solution.

The second point has a rather high priority to me as it is what I think we need for MPC with collision avoidance.

EDIT: The second point will be handled in #157

[jmirabel]

FYI @kzorina @MaximilienNaveau

[MaximilienNaveau]

The second point I disagree. We should provide such a warmstart but not as base. As an educated guess from the one picking the warmstart they want.
Hence I would suggest to implement a daughter class that implements such warmstart.

[jmirabel]

I have played with the second point using a child class of WarmStartBase. This is implemented in #124