Update LinkedList.py

[abertell]

Fixed a __getnode typo, changed __repr_\ method from O(n^2) to O(n), and implemented four new methods:

• to_list(): Converts Linked List to list in O(n).
• appendleft(value): Appends to the left in O(1).
• merge_left(other): Destructively merges another linked list on the left in O(1).
• merge_right(other): Destructively merges another linked list on the right in O(1).