Assigns indices to individuals in population.

def _initialize_pop(self, pop_size):
        """Assigns indices to individuals in population."""
        self.toolbox.register("individual", self._generate)
        self.toolbox.register("population", tools.initRepeat,
                              list, self.toolbox.individual)
        self.population = self.toolbox.population(n=pop_size)
        self.assign_fitnesses(self.population)
        self._model_count += len(self.population)
        return