Fix multithreading

[jvdd]

Decided to remove the n_threads argument and go back to the parallel argument, as there is a considerable overhead when creating a new Threadpool.
Instead, users can control the number of threads by setting the TSDOWNSAMPLE_MAX_THREADS env variable.

[codspeed-hq]

CodSpeed Performance Report

Merging #68 will degrade performances by 28.64%

_{Comparing fix_multithreading (dfcb5af) with main (7f20573)}

Summary

⚡ 12 improvements
❌ 10 regressions
✅ 368 untouched benchmarks

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Benchmarks breakdown

	Benchmark	main	fix_multithreading	Change
❌	test_minmax_no_x[True-float32-100-1,000,000]	4.9 ms	5.6 ms	-12.25%
⚡	test_minmax_no_x[True-float32-1,000-100,000]	1.5 ms	1.3 ms	+21.68%
⚡	test_minmax_no_x[True-float64-5,000-100,000]	3.4 ms	2.8 ms	+23.06%
⚡	test_minmax_no_x[True-float64-100-100,000]	1.8 ms	1.6 ms	+15.37%
❌	test_minmax_no_x[True-int32-5,000-100,000]	3.3 ms	3.9 ms	-14.85%
❌	test_minmax_no_x[True-float32-100-100,000]	1.1 ms	1.3 ms	-15.61%
❌	test_minmax_with_x[True-float64-1,000-1,000,000]	14.4 ms	18.8 ms	-23.31%
❌	test_minmax_with_x[True-int32-1,000-1,000,000]	10.6 ms	13.6 ms	-22.46%
❌	test_minmax_with_x[True-int64-1,000-100,000]	7.1 ms	9.2 ms	-22.65%
❌	test_minmax_with_x[True-int32-1,000-100,000]	6.7 ms	8.7 ms	-23.26%
⚡	test_minmax_with_x[True-int64-100-100,000]	4.1 ms	3.6 ms	+12.68%
⚡	test_m4_no_x[True-float32-5,000-100,000]	2.1 ms	1.7 ms	+21.06%
⚡	test_m4_no_x[True-float32-1,000-100,000]	1.5 ms	1.1 ms	+36.98%
⚡	test_m4_no_x[True-float32-5,000-1,000,000]	6.2 ms	5.5 ms	+11.89%
⚡	test_m4_no_x[True-float64-5,000-100,000]	2.5 ms	2.1 ms	+15.68%
❌	test_m4_no_x[True-int32-100-100,000]	1.8 ms	2.5 ms	-28.64%
⚡	test_m4_no_x[True-int32-1,000-100,000]	2.2 ms	1.8 ms	+24.46%
⚡	test_m4_no_x[True-int32-5,000-100,000]	2.8 ms	2.5 ms	+13.76%
❌	test_m4_with_x[True-float32-1,000-100,000]	5.5 ms	7.5 ms	-26.59%
⚡	test_m4_with_x[True-float32-100-100,000]	2.6 ms	2.3 ms	+15.19%
...	...	...	...	...

ℹ️ Only the first 20 benchmarks are displayed. Go to the app to view all benchmarks.