Droplet realism

[agrignard]

Right now the droplet features is implemented as follow:

species droplet skills:[moving]{
    int live_span <- droplet_livespan update: live_span - 1;
    int size<-14+rnd(200);
    aspect base{
        draw circle(size/1000) color:rgb(size*1.1,size*1.6,200,50);
    }
}

ask people{
  create droplet{
    location<-myself.location+ {rnd(-droplet_distance,droplet_distance),rnd(-droplet_distance,droplet_distance)};
  } 
}
ask droplet where (each.live_span <= 0) {do die;}

Wold be nice to have something more realist, even in 3D, @tnguyenh I think some cool mathematical equation could be useful here

In will impact also #108

[agrignard]

Duplicated with #92