Fix `mypy` errors in `lorentz_transformation_four_vector.py` (#8075)

* updating DIRECTORY.md

* Fix mypy errors in lorentz_transformation_four_vector.py

* Remove unused symbol vars

* Add function documentation and rewrite algorithm explanation

Previous explanation was misleading, as the code only calculates Lorentz
transformations for movement in the x direction (0 velocity in the y and
z directions) and not movement in any direction

* updating DIRECTORY.md

* Update error message for speed

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