cubic spline implementation

This cubic spline algorithm was described in [1] and is identical to the ones in Wikipedia [2], the shiftedbit blog [3] or the Apache Commons Math Java library. It shows a better interpolation at the borders of the spline range, compared to the Eigen spline algorithm, see screenshot.

[1] R.L. Burden, J.D. Faires, Numerical Analysis, 4th ed., PWS-Kent, 1989, ISBN 0-53491-585-X, pp. 126-131.
[2] Wikipedia http://en.wikipedia.org/w/index.php?title=Spline_(mathematics)&oldid=404494090#Algorithm_for_computing_clamped_cubic_splines
[3] shiftedbits http://shiftedbits.org/2011/01/30/cubic-spline-interpolation/

![newspline](https://cloud.githubusercontent.com/assets/6149805/2879181/8b83bf0c-d466-11e3-9ac8-b4aee30dee28.png)