When users change the entries of a .vector(), dolfin-adjoint doesn't notice

[Patrick Farrell]

Consider the changing_vector test. In it, the user accesses the .vector() of a diffusivity coefficient and changes it. This causes the adjoint to fail silently, as dolfin-adjoint doesn't know that the vector has been modified.

We should catch this (common) case: dolfin-adjoint should either work or give a useful error message. Change dolfin-adjoint to give a useful error message in this case.