Return best found solution if one is found

[mmghannam]

https://github.com/scipopt/russcip/blob/14dbea2cbd7dd87bba96a65b17c45e739c121f84/src/model.rs#L425

It can be that a solution is found but the status is not optimal; e.g. a timelimit is reached. We should instead check if a solution exists.

[mmghannam]

@yonch from your fork, I assume you will be adding a pull request soon? 😄

[yonch]

happy to! especially now I know the PR will have an interested reviewer :)

I think I should rebase, should be a few minutes.

[mmghannam]

Solved in #42