DDGnlpoisson

 [V,n,p,res,niter] = DDGnlpoisson (x,sinodes,Vin,nin,...
             pin,Fnin,Fpin,D,l2,toll,maxit,verbose)
     Solves the non linear Poisson equation
     $$ - lamda^2 *V'' + (n(V,Fn) - p(V,Fp) -D)=0 $$
     input:  x       spatial grid
             sinodes index of the nodes of the grid which are in the
                     semiconductor subdomain 
                     (remaining nodes are assumed to be in the oxide subdomain)
             Vin     initial guess for the electrostatic potential
             nin     initial guess for electron concentration
             pin     initial guess for hole concentration
             Fnin    initial guess for electron Fermi potential
             Fpin    initial guess for hole Fermi potential
             D       doping profile
             l2      scaled electric permittivity (diffusion coefficient)
             toll    tolerance for convergence test
             maxit   maximum number of Newton iterations
             verbose verbosity level: 0,1,2
     output: V       electrostatic potential
             n       electron concentration
             p       hole concentration
             res     residual norm at each step
             niter   number of Newton iterations