Improve class OurLinkedList, so that its users can readily
access a list's middle node. Readily here means that when users
instantiate an OurLinkedList object, they can access the node in
the middle of the list without starting from the head, counting how
many nodes to the end, then going back to the head, and skipping
half as many nodes forward. In fact, there should be no counting
for this improvement. Notice that lists with an even number of
nodes, do...