[MRG] replace 'f1' scorer by explicit variants

This makes the averaging options for P/R/F scorers clearer for users, and avoids users getting binary behaviour when they shouldn't (cf. #2094 where `scoring` isn't used). I think this is extra important because "weighted" F1 isn't especially common in the literature, and having people report it without realising that's what it is is unhelpful to the applied ML community. This helps, IMO, towards a more explicit and robust API for binary classification metrics (cf. #2610).

It also entails a deprecation procedure for scorers, and more API there: `get_scorer` and `list_scorers`