[FIX] fix spline derivative for cubic spline

fixed spline derivative for cubic spline
[TEST] adapt and fix tests
[FIX] remove extra bracket

- some compilers (gcc > 4.6) try to honor the extra bracket and compute
  x*x explicitly, leading to suboptimal code.
- removing the bracket is clearer and leads to better code
Merge pull request #817 from hroest/fix/spline_derivative

[FIX] fix spline derivative for cubic spline