[eigen] Non linear solver questions

[Koldo Ramirez <koldo.ramirez@xxxxxxxxx>]

Hello Thomas

I am now using your nice Non linear optimization package.

I have some questions. Do not be hard on me, I am not an expert . 

For example, look at this code:

   VectorXd x(2);
   
   hybrd_functor functor;
   HybridNonLinearSolver<hybrd_functor> solver(functor);
   solver.parameters.nb_of_subdiagonals = 1;
   solver.parameters.nb_of_superdiagonals = 1;
   solver.diag.setConstant(n, 1.);
   solver.useExternalScaling = true;
   if (solver.solveNumericalDiff(x) != 1)
      Cout() << "No convergence!";

Questions:
- What is the purpose of nb_of_subdiagonals, superdiagonals, setConstant(), useExternalScaling and another parameters?
- Are there other parameters to indicate max. number of iterations and required precision?
-
 If solveNumericalDiff() returns 1 the solver has converged 
successfully. Are there other parameters to check the quality of the 
results?
- You use a "functor" struct which calculates the equations 
and gives the  Jacobian if available. Could have been possible to use a 
pointer to a function instead?

Best regards
Koldo