DOC clarify RandomTreesEmbedding docstring
FIX P/R/F metrics and scorers are now for binary problems only by default

Scorers for different average parameters have been added.
Merge pull request #2679 from jnothman/prf_average_explicit

[MRG] Require explicit average arg for multiclass/label P/R/F metrics and scorers
MRG: Fix FeatureAgglomeration docs
FIX: Raise NotImplementedError for fit_predict