ENH add BaseSolver class, transform cd_solver and pn_solver into classes

[mathurinm]

it will be easier to pass instances of such classes to GeneralizedLinearEstimator (currently the solvers are hardcoded), and it will simplify the API.

[QB3]

Do we have to fully jit cd_solver or pn_solver? (I guess yes).
But this might be more painful to debug / profile.

[mathurinm]

In fact there is no reason to jit the solver classes, I realized. I left the get_spec and other methods, but you can see that the solver works fast without being jitted

Same for cd solver, since it's currently not jitted (only critical parts are), it should be easy to put into a class

[mathurinm]

closed by #63