Add a better callback model in the solver

Following the MR of @cmastall !59 (merged), we discussed callback system. I suggest to have a list of callbacks instead of a single one, and call each of them in order: for c in ddp.callback: c(ddp)

Then I suggest to introduce 3 callback classes:

• the logger, already existing, that make a collections of all relevant values of the solver, for later use.
• the display, that visualize the candidate trajectory after each solver iteration.
• the verbose, that print the current iterate of the solver. Currently, the verbose system is hard coded in the solver, which makes the solver function ugly. Plus the output is ugly as well. I liked the output of the devel branch, that @cmastall was proposing to insert back in refact. Please do so, but as an independent class.

Low priority task.