CLN core.groupby
Broken off from local branches doing non-CLN work.

Foreshadowing: some of the TypeErrors we are catching are being caused by `self.axis` not being an Index instead of an int in some cases.
Hello @jbrockmendel! Thanks for updating this PR. We checked the lines you've touched for [PEP 8](https://www.python.org/dev/peps/pep-0008) issues, and found:













There are currently no PEP 8 issues detected in this Pull Request. Cheers! :beers: 

##### Comment last updated at 2019-11-03 22:20:33 UTC thanks!