People working/going outside or coming from outside

[ptaillandier]

For the moment, there is just an activity "a_trade" that concerns only buildings that are not in the world (i.e. not overlapping the world). I will be more realistic to consider a place "outside" with a specific dynamic (either no possibility to get infected or a certain probability given as a parameter).

No idea if we should take into account the people that come to the area for a specific activity.