Add seed in run_law_model() and run_model()

[maximelenormand]

Add argument seed (NULL by default) in run_law_model() and run_model().

Hint for the way to handle this in java

int seed = Integer.parseInt(args[x]);

Random rand;
if (seed == null) {
rand = new Random();
} else {
rand = new Random(seed);
}

add rand in Multinomial_ function

Multinomial_ij(n, weights, sum, rand);

  random[k] = rand.nextDouble() * sumt;