Contains loops.py add (#2442)

* 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>