added sumset.py Fixes: #{6563} (#6742)

* Create sumset.py

* updating DIRECTORY.md

* [pre-commit.ci] auto fixes from pre-commit.com hooks

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* Add descriptive var names

* Update maths/sumset.py

Co-authored-by: Caeden <[email protected]>

* Update sumset.py

* updating DIRECTORY.md

Co-authored-by: github-actions <${GITHUB_ACTOR}@users.noreply.github.com>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Caeden <[email protected]>
Co-authored-by: Christian Clauss <[email protected]>