pandas转化成嵌套列表字典
[python - pandas reset_index after groupby.value_counts() - Stack Overflow](https://stackoverflow.com/questions/39778686/pandas-reset-index-after-groupby-value-counts)
[python - Export pandas to dictionary by combining multiple row values - Data Science Stack Exchange](https://datascience.stackexchange.com/questions/32328/export-pandas-to-dictionary-by-combining-multiple-row-values)
