BUG: group with multiple named results
- [x] closes #19029 
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

This bugfix gets rid of duplicated names that can be the result of groupby operations (#19029).
I opted to implement one of the ideas proposed by [toobaz](https://github.com/pandas-dev/pandas/issues/19029#issuecomment-354684340): duplicated names get suffixed by their corresponding position, i.e. ['name','name'] gets transformed into ['name0', 'name1'].

* A few testcases have been added.
* One particular testcase had to be changed ([test_crosstab_dup_index_names](https://github.com/pandas-dev/pandas/blob/master/pandas/tests/reshape/test_pivot.py))
  - This is because with the new bugfix crosstab does not yield a ValueError anymore.
