Function euler 

pub fn euler<T, F>(
    f: F,
    t_span: [T; 2],
    y0: &[T],
    options: &OdeOptions<T>,
) -> Result<OdeResult<T>>
where
    T: Float,
    F: Fn(T, &[T]) -> Vec<T>,

Solve an ODE system using the forward Euler method.

f(t, y) -> dy/dt defines the system. y0 is the initial state vector.

Examples

// dy/dt = -y, y(0) = 1 → y(t) = e^(-t)
let result = euler(
    |_t: f64, y: &[f64]| vec![-y[0]],
    [0.0, 1.0],
    &[1.0],
    &OdeOptions::default(),
).unwrap();
let y_final = result.y.last().unwrap()[0];
assert!((y_final - (-1.0_f64).exp()).abs() < 0.02);