Function bb_pct 

pub fn bb_pct(
    base_on_balls: Result<CountingStat, OmittedStatError>,
    plate_appearances: Result<CountingStat, OmittedStatError>,
) -> PercentageStat

BB%

Percentage of plate appearances that end in a walk (unintentional)

Hitters: Higher is better. Pitchers: Lower is better.