Implement methods for Part 4 Assignment 4

Implement DLList<T> getLE(T x) for Part 4 Assignment 4.
Implement testing methods for Part 5 Assignment 4

Implement testing methods to test Parts 1-4 for Part 5 Assignment 4.
Move files to assignment4 folder

Move BinarySearchTree.h, BinaryTree.h and test.cpp into assignment4
folder.