Updates population according to crossover and fitness criteria.

def _update_pop(self, pop_size):
        """Updates population according to crossover and fitness criteria."""
        candidates = []
        for ind in self.population:
            candidates.append(self._crossover(ind))
        self._model_count += len(candidates)
        self.assign_fitnesses(candidates)
        for i in range(len(self.population)):
            if candidates[i].fitness > self.population[i].fitness:
                self.population[i] = candidates[i]
        return