We will be using linked lists to create a list data structure.  To do
this, you will use a struct as the list node, with nodes connected as
a doubly linked list.  The data for the list should be a float. 

NB: A doubly linked list means that each node will store 2 pointers:
one to the previous element in the list, one to the next element.
However, do not make your list circular (have the head and tail
connected). Rather, have head->previous = null, and tail->next =
null. Also note this syntax.  To refer to the values inside a node,
which is referenced only through a pointer returned by new, you will
need to use '->'.

So, for instance, if I have 

head = new node;

then 

head->previous

will point to the previous node (should be null).

This list will be done in the C style, meaning instead of a class
(which we haven't talked about yet, and I don't want you to use), you
will be passing a pointer to the head of the list to each function.
Alternatively, you are welcome to make it a reference parameter, but I
would recommend using the pointer, as your main handle to the list
will be a pointer to the head node.

Pointers will be very important in this lab, so I recommend asking
questions if you have any problems.  Chapter 13 in the text will be a
good resource for this lab, as well.  Also, be very careful in your
pointer assignments, and try not to dereference a null pointer.

This lab is worth 75 points.

Your list should support the following operations:

AddElement:
    This function should take a value, and stick it at the head of the
    list. 
    
AddElement:
    An overloaded version of the above that should insert at a given
    numerical position (0 for  head, -1 for tail). 
    
Head:
    returns a pointer to the head of the list
    
Tail:
    returns a pointer to the tail of the list
    
GetNth: 
    returns a pointer to the Nth element of the list (leaves it in place)

Pop:
    returns value of the head of the list, then removes that element 
    
PopTail:
    returns value of the tail of the list, then removes that element
    
PopNth:
    returns value of the Nth element, then removes that element
    
SortedInsert:
    insert a value into the list in increasing sorted position    
        
SortList:
    returns a pointer to a new list which is the sorted version of a
    given list 
    
Reverse:
    reverses a given list
    
    
Write a tester file (called lab4username.cpp) which fully exercises
your data structure.  This includes making use of all the functions
listed above. 

For 25 points extra credit, write a version of SortList that sorts in
place.  In other words, no new memory should be allocated (and
deleting the old list after creating a new one doesn't count!).  You
may use whatever sorting algorithm you like (insertion sort, merge
sort, quicksort, etc.).