Function bollinger 

pub fn bollinger(period: usize, std_dev: f64) -> BollingerConfig

Create a Bollinger Bands configuration.

Example

use finance_query::backtesting::refs::*;

let bb = bollinger(20, 2.0);
let at_lower_band = price().below_ref(bb.lower());
let at_upper_band = price().above_ref(bb.upper());