Function desired_force_2d 

pub fn desired_force_2d(
    x: f64,
    y: f64,
    vx: f64,
    vy: f64,
    dest_x: f64,
    dest_y: f64,
    desired_speed: f64,
    tau: f64,
) -> (f64, f64)

Desired force driving an agent toward its destination in 2D.

f_desired = (v0 * e_hat - v) / tau

where e_hat is the unit vector toward the destination, v0 is the desired speed, v = (vx, vy) is the current velocity, and tau is the relaxation time.

Returns (fx, fy). If the agent is already at the destination (within 1e-12), returns (0, 0).