Display the current hour

[ptaillandier]

To help people to understand what happens in the model (and the behavior of people), it will be nice to have a way to see the current hour of the day (e.g. display in the overlay the current hour; play with the light (day/night), etc.).

(I do not do it myself as I am not the best in terms of design and because the resolution of my screen is not very common.)