Add lifetime to solution

[Andful]

This is my proposed solution to #138.
Had to change create_sol to admit &self instead of &mut self (because otherwise model cannot be borrowed). With this changes should make it not possible to perform use after free (with solution, clone_for_plugin seems to also allow use after free).
The tests pass otherwise.

Do of this what you want.

[Andful]

A more robust solution might be to add a lifetime directly to ScipPtr. So have OwnedScip and ScipRef<'a> instead of the consumed field.

[mmghannam]

Thank you very much @Andful! This solution works great for now and I added your suggestion as an issue to tackle later.

Fixes #138