spline - 1.2-4 main

aspline(1) interpolates an Akima-spline through a series of given points.
The Akima-spline interpolation approximates a manually drawn curve better
than the ordinary splines, but the second derivative is not continuous.
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The Akima-spline algorithm is described in "A New Method of Interpolation and
Smooth Curve Fitting Based on Local Procedures", H. Akima, J. Assoc. Comput.
Mach., 17(4), 1970, pp. 589-602. doi:10.1145/321607.321609