Pressure from a crowd of molecules

You will build the pressure of a gas from single molecules hitting one wall.

The idea. A molecule moving at $v_x$ in a box of length $L$ hits one wall $\dfrac{v_x}{2L}$ times each second. Each hit gives the wall the momentum $2mv_x$. So its average force is $\dfrac{mv_x^2}{L}$. Add the forces of all the molecules, and divide by the area $L^2$ of the wall.

Your tasks.

1. In hits_per_second, return how many times a molecule hits one wall each second.
2. In force_one, return the average force of one molecule on the wall.
3. In wall_pressure, the list vxs holds the x velocities of many molecules. Return the pressure on one wall of a cube of side L.