CLN: consolidate Series.quantile and DataFrame.quantile
- closes #10207
- Slightly modified test in tests/series/tests_analytics.py:575 to standardize behavior across Series and DataFrame
- passes `git diff upstream/master | flake8 --diff`

Collabed with @sergeibernstein on this, commenting here for alerts on any issues.
 can you rebase on master, and some PEP errors. thxs.
