Interpolation of irregularly spaced data
Fortran code by H. Akima R port by Albrecht Gebhardt
<albrecht.gebhardt@uni-klu.ac.at> aspline function by Thomas
Petzoldt <petzoldt@rcs.urz.tu-dresden.de> enhancements and
corrections by Martin Maechler <maechler@stat.math.ethz.ch>
Linear or cubic spline interpolation for irregular gridded
data