add in averaging for Stream objects (Sourcery refactored)

[sourcery-ai[bot]]

Pull Request #367 refactored by Sourcery.

If you're happy with these changes, merge this Pull Request using the Squash and merge strategy.

NOTE: As code is pushed to the original Pull Request, Sourcery will re-run and update (force-push) this Pull Request with new refactorings as necessary. If Sourcery finds no refactorings at any point, this Pull Request will be closed automatically.

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To manually merge these changes, make sure you're on the Gorkowski/issue364 branch, then run:

git fetch origin sourcery/Gorkowski/issue364
git merge --ff-only FETCH_HEAD
git reset HEAD^

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