SPL_FSTDRV returns the values of the first derivative of
the interpolating function at the points X2i. It is a double
precision array.
Given the arrays X and Y, which tabulate a function (with the X[i]
AND Y[i] in ascending order), and given an input value X2, the
SPL_INCR function returns an interpolated value for the given values
of X2. The interpolation method is based on cubic spline, corrected
in a way that interpolated value are also in ascending order