Fixes for append and delete methods in linked list classes

[lildonpancho]

• With the old code, the append method would exit on the first if statement every time because it was based on the count being equal to one. Instead, I changed it to check on index == 1 and removed it from the while loop. If this check fails, the code goes to the while loop, where it finishes when it reaches the desired node.

• I fixed the double-linked list object's logic when deleting the head node. With the old logic, when the head node is deleted, head.prev was set to None before setting self.head = current.next. This caused the head.prev to point to the old self.head node that was supposed to be deleted.

• Added an increase to the size for the append methods throughout all files.

• Changed the count attribute to the size attribute for DoublyLinkedList to stay consistent with SinglyLinkedList.