:sparkles: pass `n_threads` to downsample methods

[jvdd]

Currently, we provide a multi-threaded implementation that utilizes a boolean flag (parallel) to enable or disable parallel processing. However, this approach does not allow the user to control the number of threads used. The number of threads that is selected is determined by the available_parallellism function, which is the default behavior of Rayon.

To improve the flexibility of our implementation, we propose adding the ability for the user to specify the number of threads to be used. This would enable more fine-grained control over the execution of multi-threaded code, and also allow for benchmarking the runtime of the code in relation to the number of threads used. 👀