Function rk2 

pub fn rk2<F>(
    f: F,
    t0: f64,
    y0: Vec<f64>,
    t_end: f64,
    dt: f64,
) -> Vec<(f64, Vec<f64>)>
where
    F: Fn(f64, &[f64]) -> Vec<f64>,

Runge-Kutta 2nd order (Heun’s method).