Function estimate_initial_step 

fn estimate_initial_step(
    pos: [f64; 3],
    vel: [f64; 3],
    mu_val: f64,
    thrust: &ThrustProfile,
    rtol: f64,
    atol: f64,
) -> f64

Estimate initial step size using the approach from Hairer-Nørsett-Wanner.