add callback functions in Formulation

Hi everyone,

I'd like to play with the right hand side and solution vector of each subproblem at each iteration of the optimized Schwarz scheme.

To do so, I propose to replace all calls to _volume[idom]->solve(...) with callbacks that can be altered by the user.

Would that be an option for you? Any comments or changes are of course more than welcome :).

Cheers, Nicolas.