Add optional --seed parameter to Rho8Pointer

[jfbourdon]

Due to the stochastic nature of the Rho8 algorithm in its current implementation, it is not possible to get the same output two times from Rho8Pointer which could be an issue in research and development as it doesn't allow reproducible results. This PR adds an optional -seed parameter for cases where a result must be replicated.

It must be noted that I changed the original random number generator rand::thread_rng() by StdRng::from_entropy() in order to make it compatible with the seeded generator StdRng::seed_from_u64(). I don't think it will have any impact on the randomness of the original method, but I don't know enough about theses functions to be sure.

As, there is currently no check to make sure that a positive integer was provided to --seed, the current code will panic if a negative integer is provided.