Get a random int64 from /dev/urandom to use as a seed.

Algorithm taken from Wikipedia (https://en.wikipedia.org/wiki/Poisson_distribution#Generating_Poisson-distributed_random_variables).

RandFactory returns a newly created random number generator with a new seed.

Round randomly rounds an int either up or down, weighting the odds according to how far away from the integer it is.

is initialized to config.Cfg.Computation.Random_number_seed to avoid circular imports.