CLN: Move _aggregate and _aggregate_multiple_funcs to core.aggregation
- [ ] closes #xxxx
- [ ] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Renamed functions by removing underscores and renamed `self` argument to `obj`.
nice, comment for followon.