Function rep_vector 

fn rep_vector(
    v: &Vector,
    each: usize,
    times: usize,
    length_out: Option<i64>,
) -> Result<Vector, RError>

Helper: replicate a vector with each, times, and optional length.out.

Algorithm: first apply each (repeat each element), then apply times (repeat the whole result), then truncate/extend to length.out if given.