Log species' average fitness throughout evolution.

def save_species_fitness(self, delimiter=' ', null_value='NA', filename='species_fitness.csv'):
        """ Log species' average fitness throughout evolution. """
        with open(filename, 'w') as f:
            w = csv.writer(f, delimiter=delimiter)
            for s in self.get_species_fitness(null_value):
                w.writerow(s)