text: Title: set/update the preprocessor outside the init
  Body: 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.