Fixed Optimizer doxygen to make clear that setDifferentiatorMethod() must be called before useNumericalGradient()

Fixed doxygen documentation to make it clear that you have to call `Optimizer::setDifferentiatorMethod()` *before* calling `useNumericalGradient()` or `useNumericalJacobian()` or it won't take effect. It would be better to fix the API to make this less confusing, but at least this is an improvement.

Resolves issue #1.