Add --seed option

[ChangqingW]

Added an optional seed option to make output consistent across runs

[ChangqingW]

Looks like --threads only affects the order of the cell barcodes but the counts are otherwise the same when a fixed seed is used, maybe forcing threads = 1 is unnecessary?

    // If a seed is provided force single-threaded execution
    if let Some(seed) = args.seed {
        info!("reproducibility: using seed {seed}; forcing single-threaded execution");
        args.threads = 1;
    }