default argument for _jac in functor operator() : this way, we can use AutoDiffJacobian::operator()(x,value) exactly as the original functor
default argument for _jac in functor operator() : this way, we can use
AutoDiffJacobian::operator()(x,value) exactly as the original functor

madlib/eigen,madlib/eigen,madlib/eigen,madlib/eigen