BUG: fix+test DTA/TDA/PA add/sub Index
- [ ] closes #xxxx
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Looking forward to doing a couple of test-cleanup passes.
_data -> array edits made lgtm ping on green. ping thanks!