Consider adding a method to check feasibility

[arcondello]

We currently have a Graph::feasible() method at the C++ level.

We could expose something similar at the Python level

Something like

class Model:
    ...

    def feasible(state_index: int) -> bool:
        with self.lock():
            return all(constraint.state(state_index) for constraint in self.iter_constraints())

or could wrap the C++ method directly, but that would be a bit more involved.