Unary addition operator. All coefficients are set to zero. The constant term is stored in coefficient zero. The optional string argument is used to label the polynomial. First two arguments are standard C-type arrays of X and Y, respectively.
Third argument is number of elements.
Accepting these default values creates a natural spline. Same as the first constructor except the first two arguments are objects the Vector class defined earlier. No need to pass the number of elements since it is known in the Vector class.