Add algorithm for N-body simulation - retry (#4298)

* add n_body_simulation.py

* updating DIRECTORY.md

* Rename other/n_body_simulation.py to physics/n_body_simulation.py

* updating DIRECTORY.md

* Update build.yml

* refactor examples & add doctests

* removed type-hints from self-parameter

* Apply suggestions from code review

* Update physics/n_body_simulation.py

* Update physics/n_body_simulation.py

* Update physics/n_body_simulation.py

* Don't forget self

* Fix velocity

Co-authored-by: github-actions <${GITHUB_ACTOR}@users.noreply.github.com>
Co-authored-by: Dhruv Manilawala <dhruvmanila@gmail.com>
Co-authored-by: Christian Clauss <cclauss@me.com>