REF: implement cumulative ops block-wise
- [x] closes #19296
- [ ] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

cc @TomAugspurger 
ok with moving this later, does this need a whatsnew note / additional tests as @TomAugspurger asks? > does this need a whatsnew note / additional tests as

So far i haven't been able to write tests that fail without this.  Haven't given up. do @TomAugspurger original tests work on master? > do @TomAugspurger original tests work on master?

good point!  no, but they work here.  will update Hello @jbrockmendel! Thanks for updating this PR. We checked the lines you've touched for [PEP 8](https://www.python.org/dev/peps/pep-0008) issues, and found:







There are currently no PEP 8 issues detected in this Pull Request. Cheers! :beers: 

##### Comment last updated at 2019-12-27 20:23:03 UTC same unrelated-looking docbuild failure as #29847.  im guessing this is going to be affecting all PRs about now, will take a look thanks, looking forward to the cleanups!