Logistic peak function.
Number: 4
Names: y0, xc, w, A
Meanings: y0 = offset, xc = center, w = width, A = amplitude
Lower Bounds: w > 0.0
Upper Bounds: none
nlf_logistpk(x,y0,xc,w,A)
FITFUNC\LOGISTPK
Origin Basic Functions, Growth/Sigmoidal, Peak Functions