Added more structure methods and some quality of life changes

Overall cleaned up a lot of comments.

EasyGA:
- Code cleanup.

Population:
- Added sort_by_best_fitness
- Added parent/mating pool methods.
- Renamed some methods for consistency.

Chromosome:
- Added get_gene(index).

Parent Selection:
- Improved selection methods to use the ga.selection_probability so that the roulette selection actually works well.
- Added stochastic selection.

Survivor Selection:
- Added fill_in_random and fill_in_parents_then_random.

Crossover/Mutation:
- Cleaned up code.

src/survivor_selection/survivor_selection_methods.py

@@ -4,5 +4,24 @@ class Survivor_Selection:
     """Survivor selection determines which individuals should be brought to the next generation"""
 
     def fill_in_best(ga, next_population):
-        """Fills in the next population with the best chromosomes from the last population until the population size is met."""
-        return ga.make_population(ga.population.get_all_chromosomes()[:ga.population.size()-next_population.size()] + next_population.get_all_chromosomes())
+        """Fills in the next population with the best chromosomes from the last population"""
+
+        ga.population.set_all_chromosomes(ga.population.get_all_chromosomes()[:ga.population.size()-next_population.size()] + next_population.get_all_chromosomes())
+
+
+    def fill_in_random(ga, next_population):
+        """Fills in the next population with random chromosomes from the last population"""
+
+        ga.population.set_all_chromosomes([
+                              random.choice(ga.population.get_all_chromosomes())
+                          for n in range(ga.population.size()-next_population.size())]
+                      + next_population.get_all_chromosomes())
+
+
+    def fill_in_parents_then_random(ga, next_population):
+        """Fills in the next population with all parents followed by random chromosomes from the last population"""
+
+        ga.population.set_all_chromosomes([
+                              random.choice(ga.population.get_all_chromosomes())
+                          for n in range(ga.population.size()-len(ga.population.get_mating_pool())-next_population.size())]
+                      + ga.population.get_mating_pool() + next_population.get_all_chromosomes())