Function derivatives 

fn derivatives(
    pos: [f64; 3],
    vel: [f64; 3],
    time: f64,
    mu_val: f64,
    thrust: &ThrustProfile,
) -> ([f64; 3], [f64; 3])

Derivative of state: (velocity, acceleration). Returns (dr/dt, dv/dt) as raw f64 arrays [x,y,z] each.