Carefully handle splitting of RNG seeds

[penelopeysm]

There are several instances in the codebase where we have to create N pseudo-random number generators from one, for example:

AdvancedPS.jl/src/container.jl

Lines 121 to 136 in d0d180f

	"""
	update_keys!(pc::ParticleContainer)
	Create new unique keys for the particles in the ParticleContainer
	"""
	function update_keys!(pc::ParticleContainer, ref::Union{Particle,Nothing}=nothing)
	# Update keys to new particle ids
	nparticles = length(pc)
	n = ref === nothing ? nparticles : nparticles - 1
	for i in 1:n
	pi = pc.vals[i]
	k = split(state(pi.rng.rng))
	Random.seed!(pi.rng, k[1])
	end
	return nothing
	end

Right now, this is handled by split, which generates new seeds to be used with new PRNGs.

AdvancedPS.jl/src/rng.jl

Lines 33 to 42 in d0d180f

	"""
	split(key::Integer, n::Integer=1)
	Split `key` into `n` new keys
	"""
	function split(key::Integer, n::Integer=1)
	T = typeof(key)
	inner_rng = Random.MersenneTwister(key)
	return rand(inner_rng, T, n)
	end

This is not always a reliable method as the new seeds may end up accidentally generating highly correlated sequences of numbers. See, e.g., https://gee.cs.oswego.edu/dl/papers/oopsla14.pdf

We should therefore switch to an underlying RNG type that supports splitting. (Note that right now, we are using Philox2x [paper, library] but the library does not implement a split function, even though it should theoretically be possible. Compare e.g. with Numpy's implementation, which provides a jumped method for this purpose.)