ENH: Use Kahan summation to calculate groupby.sum()
- [x] closes #38778 
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

This simplifies the op example.

I think 
```
series = Series([1e16, 99, -5e15, -5e15])
series.sum()
```
is dispatched to numpy? The result is wrong too. ``100.0``

Mean doesn't work either. I will look through the functions to determine which need Kahan summation too and open an issue to track these.