function p=DDGhole_driftdiffusion(psi,x,pg,n,ni,tn,tp,up)
DESCRIPTION
p=DDGhole_driftdiffusion(psi,x,pg,n)
Solves the continuity equation for holes
input: psi electric potential
x spatial grid
pg initial guess and BCs for hole density
n electron density (to compute SRH recombination)
output: p updated hole density