str.split expand by row
#### Problem description
Currently, setting expand=True in str.split just creates new columns. 
While there is a solution [here](https://stackoverflow.com/questions/47571618/how-to-split-expand-a-string-value-into-several-pandas-dataframe-rows) to expand row-wise, it would be much more convenient if we could specify either 'rows' or 'columns' to the expand kwarg.