Make rayon optional

[bminixhofer]

rayon is not necessary and should be behind a feature flag (like in e.g. ndarray). Resolving this issue in rayon-cond would make it trivial: https://github.com/cuviper/rayon-cond/issues/3

If this change does not get accepted in rayon-cond I believe the best solution is forking it, adding a rayon feature flag and also moving the abstraction currently in src/utils/parallelism.rs to the fork.