Lower scheduling priority of preprocessing

[pflanze]

Preprocessing, while running, takes a lot of CPU, and leads to the api becoming unresponsive at times; make sure the kernel gives the api priority by de-prioritizing the preprocessing explicitly. This can be done by starting the preprocessing via the "nice" command, which add 10 points to the priority (making it less prioritized); best run it through "nice" twice, then it reaches the maximum value of 19. I.e. have the Docker container start "nice nice path/to/silo preprocessing ...".

This could also be doable by telling docker to start the whole image with a lower priority, but then that requires us to learn how to, and it will require all users to think of it; probably better to just get it done here.