[igraph] increase network size with poisson probability

[Simone Gabbriellini]

Hello list,

right now I have a model in which nodes are added with uniform probability p. I 
have implemented this in a classical way:

if random() < prob_value:
        add_the_node

I would like to change this mechanism to reflect poisson probability, but I 
need a hint on how to do this. I see there's no random.poisson, but maybe the 
random.expovariate(lambd) can do the job:

if random.expovariate(lambd) < prob_value:
        add_the_node

best,
Simone