Sigmoidal Weibull function, type 2.
Reference: Seber, G. A. F., Wild, C. J. 1989. Nonlinear Regression. John Wiley & Sons, Inc. pp. 338 - 339
Number: 4
Names: a, b, d, k
Meanings: a = top asymptote, b = unknown, d = power, k = coefficient
Lower Bounds: a > 0.0, b > 0.0, k > 0.0
Upper Bounds: none
nlf_sweibull2(x,a,b,d,k)
FITFUNC\WEIBULL2.FDF
Growth/Sigmoidal