Add a global variable to set number of parallel threads

[LSchueler]

As pointed out by pavlovc2 in discussion #333, it would be helpful to set the number of parallel threads in GSTools directly and not only rely on environment variables.

I didn't have a lot of time to put this together, but in this first draft, you can set the number of threads with the global variable config.NUM_THREADS. At the moment, this only works with the Cython code and not with the Rust code. And also, for now, the default value of threads is 1.

TODO

• also set the number of threads for the Rust code
• find out how to set num_threads=None in the Cython code

[LSchueler]

The Rust package GSTools-core is now ready for setting the number of parallel threads and I'm quite happy with the implementation.

For Cython, I used a bit of an ugly function to set defaults, without having to use Python variables.

I think the only thing left would be to update the changelog, if the review gets through ;-)

[MuellerSeb]

See #337 for rtd issue.

[MuellerSeb]

This looks promising. Was also winding my head around the fact, that prange uses None as default value.. this is idiotic to use a python type here. We should create an issue, that prange takes 0 or -1 to mimic the None behavior.
The function you used is ugly but I see that it's the best option at the moment.

Is it testable how many cores are used during a function call? This could be cool to check if the setting is actually working.

[LSchueler]

I mean, we could get the numbers of cores in config.py and set it there and use num_threads=1 as the default values in the Cython and Rust functions. But it feels cleaner to get and set the default values in the Cython and Rust functions.

Regarding the testing, I think it's difficult to accurately test the number of threads used, if we don's simply want to check for the num_threads variable we just set.
I checked on my machine and the compute times decrease nearly linearly with the number of threads used (for num_threads < 13 at least). So, locally it works. But I'm not sure how many threads we can use in Github actions and if we really want to check the run times of different function calls.

[MuellerSeb]

Created an issue in cython for prange: cython/cython#5952