OQMD-mix, MP-mix, ICSD-mix datasets

Datasets with mixed samples, some are charge neutral, some are not. Some are electroneativity balanced. Some are not. <br>本数据集包含混合样本，部分样本为电荷中性（charge neutral），其余样本不满足电荷中性条件；另有部分样本的电负性（electronegativity）处于平衡状态，剩余样本未达到电负性平衡要求。