Rename survival_percentage to something clearer.

right now `population_size` is the number of generated offspring. `survival_percentage`*`population_size` is the number of parents to keep each generation. `survival_percentage` was developed with successive halving in mind, with the assumption that we want to scale the parent size with the population size.

We could rename `survival_percentage` to parent_size, and have successive halving scales the parent size proportional to population_size:parent_size