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.
Merge pull request #2 from sherm1/master

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