remove some duplicated code LevenbergMarquardt::minimizeNumericalDiff*() by

using the generic Eigen NumericalDiff recently introduced.

LevenbergMarquardt::lmdif1(), which is provided as a convenience method for
people porting code from (c)minpack, is now a static function

unsupported/Eigen/NonLinear

@@ -26,6 +26,7 @@
 #define EIGEN_NONLINEAR_MODULE_H
 
 #include <Eigen/Core>
+#include <unsupported/Eigen/NumericalDiff>
 
 namespace Eigen {