WIP: prototype for unit support #10349 
Trying to follow @jreback idea of adding a dtype for holding the unit.

Related to issue #10349 
- [ ] closes #xxxx
 - [ ] tests added / passed (incomplete tests)
 - [ ] passes ``git diff upstream/master -u -- "*.py" | flake8 --diff`` (except for one warning)
 - [ ] whatsnew entry
