TODO Let people healing with a probability (decreases exponentially?) 
over time, similar probability to die and 1-p_die-p_heal
to be still sick

Simulation details:

2D grid spacexspace representing the space

Each cell of the grid can be contaminated or not contaminated. 
After dt timesteps a cell is contaminated comes back to be not 
contaminated

It starts from a population of npeople people with one ninfected.

People are moving through random walks (dispacement defines the 
size of the motion)

If an infected person passes through a not contaminated cell the 
cell becomes contaminated, if it's already contaminated the timer of the contamination start over

If a person not infected passes through a contaminated cell has a 
probability of p_get_infected to get infected

After lifetime timesteps a person is infected is deleted from the 
list of people (and from the grid)