Function desired_force_3d 

pub fn desired_force_3d(
    x: f64,
    y: f64,
    z: f64,
    vx: f64,
    vy: f64,
    vz: f64,
    dest_x: f64,
    dest_y: f64,
    dest_z: f64,
    desired_speed: f64,
    tau: f64,
) -> (f64, f64, f64)

Desired force driving an agent toward its destination in 3D.

Same formulation as desired_force_2d extended to three dimensions.

Returns (fx, fy, fz).