Function adams_bashforth 

pub fn adams_bashforth(
    f: &dyn Fn(f64, &[f64]) -> Vec<f64>,
    x0: f64,
    y0: &[f64],
    x_end: f64,
    n: usize,
) -> (Vec<f64>, Vec<Vec<f64>>)

Adams-Bashforth 2-step for systems.