* add DataSet.get_field(), to fetch a FieldArray based on its name * remove old tutorials & add new tutorials
* 更新测试代码,跑通所有测试,覆盖率65% * refine代码规范和某些注释 * fix tester self.use_cuda未赋值先使用的bug * 添加tutorial样例数据——tutorial_sample_dataset.csv * 【unsolved】embed_loader在计算np.cov时遇到segmentation fault