iFit/powerlaw

 y = powerlaw(p, x, [y]) : power law model

   iFunc/powerlaw power law fitting function
     y = p(1)*(x - p(2))^p(3) + p(4);

 poisson(centre)         creates a model with a specified centre
 poisson([ parameters ]) creates a model with specified model parameters

 input:  p: power law model parameters (double)
            p = [ Amplitude Centre Exponent BackGround ]
          or 'guess'
         x: axis (double)
         y: when values are given and p='guess', a guess of the parameters is performed (double)
 output: y: model value
 ex:     y=powerlaw([1 0 1 1], -10:10); or plot(powerlaw);

 Version: Aug. 22, 2017
 See also iFunc, iFunc/fits, iFunc/plot
 (c) E.Farhi, ILL. License: EUPL.