Get the per-generation average cross_validation fitness.

def get_average_cross_validation_fitness(self): # pragma: no cover
        """Get the per-generation average cross_validation fitness."""
        avg_cross_validation_fitness = []
        for stats in self.generation_cross_validation_statistics:
            scores = []
            for fitness in stats.values():
                scores.extend(fitness)
            avg_cross_validation_fitness.append(mean(scores))

        return avg_cross_validation_fitness