Function moonlander_gp::retain_best

pub fn retain_best<P, F, R>(
    frac: Number, 
    pop: Population<P, F>, 
    max_depth: usize, 
    rng: &mut R
) -> Population<P, F> where
    P: RandNode + Clone + Sync + Encodable,
    F: Fitness + Sized + Send + Encodable,
    R: Rng, 

Take the best fraction of the population and fill back up to N with random programs.