Fix the random number generator

As discussed in issue #545 (closed) the random number generator is Swift is currently poor. In this merge request I basically aim at solving the issue with correlated random numbers and make the random numbers uncorrelated and added two tests:

1. First test checks if the random numbers are correlated in time.
2. Second test checks if the random numbers are correlated in adjacent IDs at fixed times.

Let me know what you think of the code. I will do some final testing on the isolated galaxy to make sure everything works.