A=Uscharfettergummel(nodes,Nnodes,elements,Nelements,acoeff,bcoeff,v)

Builds the Scharfetter-Gummel  matrix for the
the discretization of the LHS
of the Drift-Diffusion equation:

$ -(a(x) (u' - b v'(x) u))'= f $

where a(x) is piecewise constant
and v(x) is piecewise linear, so that
v'(x) is still piecewise constant
b is a constant independent of x
and u is the unknown