ENH: Explode multiple columns of DataFrame
Now .explode() can take a list of column names and will explode multiple at the same time (given that each element across all the columns have the same length in every single row
- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry
