Handling high-order differentiation of forms with FEMExternalOperator-s

[a-latyshev]

FEMExternalOperator-s could be used to define variational formulations that requires high-order differentiation in an elegant way:

psi = FEMExternalOperator(u, function_space=Q, name="psi")
E = psi * dx 
F = ufl.derivative(E, u, ufl.TestFunction(V))
J = ufl.derivative(F, u, ufl.TrialFunction(V))

This is bugged due to the same issue in the original UFL ExternalOperator: FEniCS/ufl#449