Use NguyenWidrowRandomizer in BasicNetwork.reset(seed)

Thanks to Robert Coop's change 61ebdaf0c1c30426d314a1b5a5a4875edadae6b we can now use the NguyenWidrowRandomizer with a given seed. The NguyenWidrowRandomizer had been removed from BasicNetwork.reset(seed) in 884a95eaaf3f4cdfa6a39f0c8327d6edd5fb8f68 because at that time it didn't use seeds.

RangeRandomizer and NguyenWidrowRandomizer now both work with the setSeed method, so the choice of the Randomizer doesn't depend any longer on whether or not a seed is given and can be unified in a private function getRandomizer().