Random number generator as argument

[bicycle1885]

There are several rand function calls and friends that depend on the global random number generator. This is okay, but it is not be a standard way of implementing simulation code because multithreaded code will need more control on random number generation for reproducibility. The standard way is passing a random number generator (RNG) as the first argument of a function, as in rand of Base. I think we also should follow this convention so that users can easily find a way to control it.