Center window
- [x] closes #20012
- [ ] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Added center functionality for `VariableWindowIndexer`. Note - I am unsure if the NotImplementedError in lines 1966-1969 in rolling.py still correctly raises an error for offset based windows.
This pull request is stale because it has been open for thirty days with no activity. Please update or respond to this comment if you're still interested in working on this. Closing for now. @adamamiller ping whenever you'd like to continue and we'll reopen! Would love to see this functionality and would like to pick up the pull request with @luholgit

What are the next required steps?

We already forked the pandas repo and checked out the branch from @adamamiller.

However, when trying to update the branch by pulling from origin/master into fork/center_window we encountered several merge conflicts which we are not sure how to solve.

Should we solve them based on our understanding and then discuss those changes in this PR?



 @jreback and @arw2019, mentioning you explicitly as we need this change urgently and would love to implement at the open source directly as opposed to our private repository! :) you see the comments above

this needs to have master merged and needs tests  Thanks for your quick response, we are implementing the merge and tests right now.

(Un)Fortunately, we were not able to push to @adamamiller's fork. 

Could you change the pull request to come from https://github.com/luholgit/pandas/tree/center_window?
 Or shall we create a new issue such that the new source branch from our fork is selected automatically? @Zaubeerer thanks for taking this forwards - you can open a new pull request @Zaubeerer you just create a new PR, no need to create a new issue