Rename survival_percentage to something clearer.

[perib]

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