Retrieves the knot vector in the V direction. 

A knot vector defines a sequence of parameter values that determine where and how the control points affect the surface. Values of knot vector are provided in ascending order, for example: (0, 0, 0, 1, 2, 3, 3, 3) can be valid while (0, 0, 2, 0, 3, 1, 3, 3) cannot be valid.