SG=Ufastscharfettergummel2(mesh,v,acoeff,bcoeff)


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

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

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