Update Population.java

New methods: addNewSubpopulation (adds a Subpopulation to subpopulations and size_by_id, adds 1 to n_subpopulations), getSubpopById (returns a Subpopulation with the given id or null if it doesn't exist), getSubpopsByType (returns a list of subpops with either "coop" or "cheat" in the beginning of its id).