add to_frame method to DataFrame
Add convenience method which makes sure that we can safely call `to_frame` on any method returning both `Series` and `DataFrame` instances, such as `groupby` etc.

- [ ] ~~closes #11292 xxx~~ (there is no issue, should I open one?)
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry
