Function rng_for 

pub fn rng_for(study_seed: u64, trial_number: u64, stream: Stream) -> ChaCha8Rng

Builds the pinned RNG for one random stream of one trial.

Equivalent to rng_from_seed(seed_for(study_seed, trial_number, stream)).

use atune_core::seed::{Stream, rng_for};
use rand::RngExt;

let mut a = rng_for(1, 0, Stream::Sampler);
let mut b = rng_for(1, 0, Stream::Sampler);
assert_eq!(a.random::<u64>(), b.random::<u64>());