Change RNG to PCG64

[mnwright]

As noted in #414, another random number generator (RNG) might be better than std::mt19937_64. Here, we switch to PCG.

TODOs:

• Check whether this is faster (or slower)
• Check reproducibility: Looking at the tests on Github actions, this won't fix the different results on Mac (Results are different between on Mac and on Windows #533).
• Double check license issue. I think, including it as such is fine.
• Are there any other side effects?