Possibility to limit number of used CPU cores?

[BafDyce]

My machine has 24 logical cores. All of them spin up and run a workload, even for the simplest application. It would be great if there was a possibility to limit the number of actively used cores. This would have the following benefits:

• Dont waste power/dont heat up your room when you're only doing basic stuff
• Dont impact other applications in the background too much
• Allows to do benchmarks for lower end systems without having to spin up VMs / containers with cgroups.

This is probably also related to #111

I was already poking around in ParallelExecutor but couldnt identify any obvious place where this could be added.

[Ratysz]

This is due to rayon defaulting to as many threads as there are cores when initializing it's global thread pool. Perhaps resource+behavior akin to the window settings can be implemented to tune the global thread pool without requiring users to invoke rayon directly?

[karroffel]

#188 was merged, so this can be closed 🎉