feat: Add mass energy equivalence in physics and doctests (#10202)

* updating DIRECTORY.md

* feat: Add mass energy equivalence in physics

* updating DIRECTORY.md

* updating DIRECTORY.md

* Apply suggestions from code review

* Update physics/mass_energy_equivalence.py

* Update mass_energy_equivalence.py

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Co-authored-by: github-actions <${GITHUB_ACTOR}@users.noreply.github.com>
Co-authored-by: Tianyi Zheng <[email protected]>