NewtonSolver without call to NonlinearProblem.form

[Henning Bonart]

In dolfin's NewtonSolver::solve(), the following calls are performed:

// Compute F(u)
nonlinear_problem.form(*_matA, *_matP, *_b, x);
nonlinear_problem.F(*_b, x);

, see https://github.com/FEniCS/dolfin/blob/d09c0bd4648c9c80996f929c97da38d2dec44999/dolfin/nls/NewtonSolver.cpp, line 128.

In dolfin-adjoint's NewtonSolver.solve(), only this call is performed: factory.F(b=b, x=vec). This fails, if only the NonlinearProblem.form performs the assembly, see the comment on line 57 at https://github.com/FEniCS/dolfin/blob/d09c0bd4648c9c80996f929c97da38d2dec44999/dolfin/nls/NewtonSolver.cpp.

Changing factory.F(b=b, x=vec) to

factory.form(None, b, vec) 
b = factory.b

seems to work.