* added an algorithm which checks a linked list for loops and returns true if one is found
* added doctests and clarified meaning of loop
* Define Node.__iter__()
* Update and rename has_loop.py to has_duplicate_data.py
* Update has_duplicate_data.py
* Update has_duplicate_data.py
* Update and rename has_duplicate_data.py to has_loop.py
* Update has_loop.py
Co-authored-by: Christian Clauss <cclauss@me.com>