title: Can openmdao compute partial derivatives across a Matlab ExternalCodeComp without explicitly defining them?
body: <p>Is it possible to have openmdao to approximate partial the derivatives across the ExternalCodeComp using finite difference.</p>
  <p>Just by using the method <code>self.declare_partials('*','*', method='fd')</code> seems not to work
  Optimization converges after 1 iteration with only 1 function and gradient evaluations.</p>
  <p>The error that pop ups :
  DerivativesWarning:Constraints or objectives [('p.f_xy', inds=[0])] cannot be impacted by the design variables of the problem.
  DerivativesWarning:Design variables [('p.x', inds=[0]), ('p.y', inds=[0])] have no impact on the constraints or objective.
  Optimization terminated successfully .</p>