The file `getfem/getfem_derivatives.h` defines the following function to
compute the gradient of a solution:

```
getfem::compute_gradient(mf1, mf2, U, V);
```

where `mf1` is a variable of type mesh_fem and describes the finite element method
on which the solution is defined, `mf2` describes the finite element method to
compute the gradient, `U` is a vector representing the solution and should be
of size `mf1.nb_dof()`, `V` is the vector on which the gradient will be
computed and should be of size `N * mf2.nb_dof()`, with `N` the dimension of
the domain.