Function extract_features 

pub fn extract_features(
    buf: &mut FeatureBuffer,
    config: &SeqFeatureConfig,
    observations: &[&str],
    i: usize,
    labels: &[&str],
)

Extract AP features into buf for position i.

labels are the predicted labels so far (from the left-to-right pass). Label features use negative positions to look back at previous labels.