/*
 *  call-seq:
 *     Dvector.create_pm_cubic_interpolant(xs, ys) ->  interpolant
 *
 *  Uses Dvectors _xs_ and _ys_ to create a cubic pm_cubic interpolant.  The _xs_ must be given in ascending order.
 *  The interpolant is an array of Dvectors: [Xs, Ys, As, Bs, Cs].
 *  For x between Xs[j] and Xs[j+1], let dx = x - Xs[j], and find interpolated y for x by
 *  y = As[j]*dx^3 + Bs[j]*dx^2 + Cs[j]*dx + Ys[j].
 *  pm_cubic algorithms derived from Steffen, M., "A simple method for monotonic interpolation in one dimension", 
 *        Astron. Astrophys., (239) 1990, 443-450.
 *  
 */ 
VALUE dvector_create_pm_cubic_interpolant(int argc, VALUE *argv, VALUE klass) {