* 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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