[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