Add to Tail

Time to allow our LinkedList to add new nodes to the end of the list. Kind of like a regular Python List's .append method.

Assignment

Complete the add_to_tail method. It adds a new node to the end of the list and returns nothing.

If there isn't a head node, set the new node as the head and be done.
Otherwise, keep a reference to the "last" node in the list - start with it set to None.
Iterate over the linked list (you can use a for loop now that you've added your own __iter__!)
- Update your "last" node reference to the current node
Once you've iterated over the entire list, your "last" reference should be the last node in the list (the "tail"). Set the next field of the "last" node to the new node.