Able to add constraints

[Wikunia]

At the moment it's not really possible to add constraints. This would need to delete all previously feasible but now infeasible solutions. If a prev feasible solution is still feasible it can be returned otherwise a new search needs to be created but prev infeasible solutions are infeasible therefore they can be ignored.

[Wikunia]

For the next version it's sufficient to disallow adding new constraints by throwing an error if optimize was called already.