Fancy function derivative features

Functions (low priority, could allow automatic linearization w.r.t. functions not in a finite element space?):
    - (low) Attaching derivatives to Functions:
      It would be nice if the user could supply the derivative of a Function as another Function.
      Maybe we can do:

        Df = Function(element2)
        f = Function(element1, gradient=Df)

      and perhaps for linearization:

        g = Function(element3)
        df_dg = Function(element2)
        f = Function(element1, derivatives = { g: df_dg })

      then you could do:

        F = derivative(f**2*dx, f)
        J = derivative(F, f)

      even if f is defined on a quadrature element.