DOC: got rid of shouting comment
ENH: error if no closing "% <<" found
RF: proper using of singular
DOC: improved documentation for winner_indices
ENH: more difficult dataset for testing
RF+BK: use dataset_slice as a general function rather than two seperate ones ("dataset_slice_features" and "dataset_slice_samples")
DOC: note in "dataset_slice_{f,s}a that dataset_slice is a preferred function to use