Touches nearly everything unfortunately but is necessary.

Constraint parameters Intensity, Diffusion, and Radius
to positive values

 - I left derivative as undefined. Thus, the "Simplex" algoritm
   is used. Test minimization takes less than one second

- helper functions initXnlCoeff, initAlphaCoeff, initLinJlist,
         LorentzianCoefficients

 - lookup tables alpha and linearJlist.
 - numerical interpolation when close to numerical divergence
   for factor J.

and some minor enhancements.

The old one was back to front.

Actually there are tests which should test main logic. Not fully comprehensive, but enough for the time being.

A bit more tests would be useful.

reflected in gui and in algorithm itself