Use only a sample of `sample_pct`of the full dataset and an optional `seed`.

def use_partial_data(self, sample_pct:float=0.01, seed:int=None)->'ItemList':
        "Use only a sample of `sample_pct`of the full dataset and an optional `seed`."
        if seed is not None: np.random.seed(seed)
        rand_idx = np.random.permutation(range_of(self))
        cut = int(sample_pct * len(self))
        return self[rand_idx[:cut]]