* Reimplement polynomial_regression.py
Rename machine_learning/polymonial_regression.py to
machine_learning/polynomial_regression.py
Reimplement machine_learning/polynomial_regression.py using numpy
because the old original implementation was just a how-to on doing
polynomial regression using sklearn
Add detailed function documentation, doctests, and algorithm
explanation
* updating DIRECTORY.md
* Fix matrix formatting in docstrings
* Try to fix failing doctest
* Debugging failing doctest
* Fix failing doctest attempt 2
* Remove unnecessary return value descriptions in docstrings
* Readd placeholder doctest for main function
* Fix typo in algorithm description
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