CLN: Some groupby internals
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

1. Avoids calling `_get_sorted_data()` twice
2. Eliminates `*args, **kwargs` from an internal function
Looks like a segfault on travis - restarted to see potential annotations; LGTM pending green Can you merge master? Slightly concerned about reproducibility of previous segfault Thanks for the review. Merged master and all green