[FR] Add nthreads argument to CPU backend

[cncastillo]

Hi! First of all congratulations on the fantastic package :)

Sometimes, seeing how the speed scales as a function of the number of threads is helpful.

My understanding is that right now the only way to change this is to run Julia differently julia --nthreads=X, and KA uses Julia's Threads.nthreads() (L95 here) to launch different processes:

KernelAbstractions.jl/src/cpu.jl

Lines 93 to 100 in bafa3cc

	function __run(obj, ndrange, iterspace, args, dynamic, static_threads)
	N = length(iterspace)
	Nthreads = Threads.nthreads()
	if Nthreads == 1
	len, rem = N, 0
	else
	len, rem = divrem(N, Nthreads)
	end

Could this be an argument of the CPU backend? For example, launching a kernel with CPU(8) would run with 8 threads (if 8 <= Threads.nthreads()). So we don't need to restart Julia with a different number of threads.

Obviously it doesn't need to be an argument in the backend, there might be a better way to implement this

Forgive me if there is a way to do this that I haven't realized.

Cheers!