Expose as get_next_best_solution function

[oxinabox]

Given #41 would mean there is a single best solution
we should expose a way for the user to request the next best solution.
Which would under the hood just call forbid_solution to block the current one, and then re-solve it.

I think this is useful to end-users, since they might be like "this one just doesn't look right, what is the next best candidate"?