Extend Hyperparameter Run

[mo-fu]

Hypeparameter optimization may run for a long time. If I optimize an ensemble I first have to optimize the individual parts and then the ensemble afterwards. If I want to get a result quickly I may limit the number of trials. However, if I have additional compute capacity in the future I may want to run another hyperparameter optimization without discarding the previous run. Optuna studys should be serializable. So they can be loaded again and extended with additional trials.

[osma]

Sounds like a nice feature, PRs welcome!