[a,b] = FitLogistic(x,y)
Fit a logistic function to the data pairs x,y.
The form of the logistic equation is y = 1/(1+10^-(ax+b)).
The method used here is to regres on transformed coordinates.
One could use search to do a maximum likelihood function for
psychometric data, but if you are going to do that, you
should probably fit a cumulative normal or Weibull function.
2/15/95 dhb Wrote it.