BUG: 43909 - check monoticity of rolling groupby
- [x] closes #43909 
- [x] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit) for how to run them
- [x] whatsnew entry
Whats new:

- Exception now raised when non-monotonic data passed to rolling groupby
- Ammendment of tests to reflect this changed behaviour
- Addition of test for the new behaviour 

Hello @jamesholcombe! 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 2021-12-22 13:40:13 UTC @github-actions pre-commit This PR should close issue 43909. Previous behaviour of a rolling window on a groupby was inconsistent with behaviour of rolling window on a dataframe. The former raises an error for non-monotonic "on". I have amended the latter to match this behaviour. @github-actions pre-commit @github-actions pre-commit Whats New: Value Error now raised when a non-monotonic time series passed to "_on" attribute of a rolling groupby. This is to match the error raised for a rolling window on a dataframe. Tests have been added to confirm expected behaviour. @github-actions pre-commit @github-actions pre-commit Hi Jeff, 
master is merged no? 
which sections require commenting?
Cheers @github-actions pre-commit
 NOTE: master branch contains breaking changes so have not merged most recent. > NOTE: master branch contains breaking changes so have not merged most recent.

what does this mean?

 still needs a whatsnew note and merge master still needs a whatsnew note and merge master @github-actions pre-commit @github-actions pre-commit Find whats new note at top of PR. master merged. @github-actions pre-commit whatsnew note goes in the doc/ folder under 1.4.0 for bug fixes pre-commit is failing, pls merge master and ping on green. pre commit now passing thanks @jamesholcombe 