set/update the preprocessor outside the init

[wdevazelhes]

Just a thought, but it might be useful to have a function to update/set the preprocessor outside of the init. Right now it is just initialized when calling fit (so using sth like mcc.set_params(preprocessor=X) won't help). But one might realize later after fitting that they need a/another preprocessor, for instance if they want to score_pairs on a huge dataset, or maybe if they want to score_pairs on a different dataset than the one they used at fit time.

[bellet]

I think we thought about that and this is why we still allow the user to submit pairs in feature form instead of id even when a preprocessor has been initialized

But we could add such an option for convenience