data_structures/linked_list: Add __len__() function and tests (#2047)

* Update __init__.py

please add a function to get length of linked list

* Update __init__.py

* Update doubly_linked_list.py

all size function lo doubly linked list class

* prime number _better method

* comments

* Updated init.py 2

made it more pythonic

* updated length function

* commnet in linked_list construtor

* Update data_structures/linked_list/__init__.py

accepecting changes

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

* Update data_structures/linked_list/__init__.py

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

* Update __init__.py

* Revert changes to doubly_linked_list.py

* Revert changes to prime_check.py

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