Use `concurrent.futures` module for running things in parallel

[bouweandela]

The simplest way to run something in multiple threads/processes is through the concurrent.futures module instead of 'manually' managing the threads/processes as currently done in https://carpentries-incubator.github.io/lesson-parallel-python/threads-and-processes.html. Therefore it might be nice to

Therefore, it might be nice to use the concurrent.futures module instead.

[jhidding]

We use the chapter on Threads and Processes to explain the conceptual difference between those and talk about the GIL. concurrent.futures is an abstraction that belongs in the same ball park as dask delayed. If it is about ease of use, we can demo the concurrent.futures.ThreadPoolExecutor as a replacement for multiprocessing.ProcessPool. How often are we really using ThreadPool as a means by itself?