NLP Word Frequency Algorithms (#2142)

* NLP Word Frequency Algorithms

* Added type hints and Wikipedia link to tf-idf

* Update machine_learning/word_frequency_functions.py

Co-authored-by: Christian Clauss <cclauss@me.com>

* Update machine_learning/word_frequency_functions.py

Co-authored-by: Christian Clauss <cclauss@me.com>

* Update machine_learning/word_frequency_functions.py

Co-authored-by: Christian Clauss <cclauss@me.com>

* Update machine_learning/word_frequency_functions.py

Co-authored-by: Christian Clauss <cclauss@me.com>

* Fix line length for flake8

* Fix line length for flake8 V2

* Add line escapes and change int to float

* Corrected doctests

* Fix for TravisCI

* Fix for TravisCI V2

* Tests passing locally

* Tests passing locally

* Update machine_learning/word_frequency_functions.py

Co-authored-by: Christian Clauss <cclauss@me.com>

* Update machine_learning/word_frequency_functions.py

Co-authored-by: Christian Clauss <cclauss@me.com>

* Update machine_learning/word_frequency_functions.py

Co-authored-by: Christian Clauss <cclauss@me.com>

* Update machine_learning/word_frequency_functions.py

Co-authored-by: Christian Clauss <cclauss@me.com>

* Add doctest examples and clean up docstrings

Co-authored-by: Christian Clauss <cclauss@me.com>