// Precalculate 2nd derivative of y and put in m_pdY2
// Calculated by solving set of simultaneous CubicSpline spline equations
// Only n-2 CubicSpline spline equations, but able to make two more
// Precalculate 2nd derivative of y and put in m_pdY2
// Calculated by solving set of simultaneous CubicSpline spline equations
// Only n-2 CubicSpline spline equations, but able to make two more