Function apply_moving_wall_3d 

pub fn apply_moving_wall_3d(
    sim: &mut PyLbm3dSimulation,
    z_layer: usize,
    vel: [f64; 3],
)

Apply a moving-wall (Zou-He) velocity boundary to a row of cells.

Forces the top row (z = nz-1) of a 3-D grid to have velocity vel.