BUG: DTI/TDI intersection result names
- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry


Also improve _can_fast_intersect to catch the missing case where we can fast-intersect.  Identified that _most_ of the remaining places where we do freq inference are not tested, so may be removeable/deprecateable