M4 Multithreading fails when using 16 cores

From commit https://github.com/predict-idlab/tsdownsample/pull/51/commits/4afdbf5b361818b286b7ee1727bb9636034dc584 onwards, the M4 algorithm's random sequence test can fail when using 16 cores. This is probably due to a mathematical error somewhere in one of the functions that gets called when performing multi threaded M4.