BUG: Groupby.cummin/max DataError on datetimes (#15561)
 - [x] closes #15561
 - [x] tests added / passed
 - [x] passes ``git diff upstream/master | flake8 --diff``

`numeric_only=False` set by default for `groupby.cummin/cummax` like `groupby.max/min` allows datetimes to work. 