[bugfix] Reimplement accidentally removed boundary function

test/hyperbolic/lineartransport.ufl

@@ -23,7 +23,7 @@ mass = u*v*dx
 r = -1.*u*inner(beta, grad(v))*dx \
   - numerical_flux(n, u('+'), u('-'))*jump(v)*dS \
   + inner(beta, n)*u*v*dso \
-  + numerical_flux(n, g, u('-'))*v*dsd
+  + numerical_flux(n, 0.0, u('-'))*v*dsd
 
 forms = [mass, r]
 is_dirichlet = dirichlet