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
Fortran code: ACM, free for non-commercial use, R functions GPL