This lab just impliments a linked list that does not allow duplicates. has the following methods:
insertHead(value) adds a value at the beginning of the list
insertTail(value) adds a value at the end of the list
insertAfter(value, insertionNode) adds a value after a specified value in the list, if it exists (insertinNode)
remove(value) removes a given value from the list if it is in the list already
clear() clears the list
at(index) returns the value at a given index
size() returns the length of the list
toString() converts each value to a string and concatenates