ENH: add GroupBy.pipe method
- [x ] closes #10353, see also #17863
- [x ] tests added / passed
- [ x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ x] whatsnew entry

This PR adds a ``.pipe`` method to GroupBy objects like for ``DataFrame.pipe`` and ``Series .pipe``.

This PR is basically #10466 written by @ghl3 with some very minor updates, because that PR somehow got stalled and subsequently was closed.

This PR says it's new in 0.21, but I'll change that if it's too late to add this.