[feature] Fix a pb in DEBUG mode when computing error dot.

Found by Fanny Risbourg

include/sot/core/feature-point6d.hh

@@ -126,7 +126,7 @@ class SOTFEATUREPOINT6D_EXPORT FeaturePoint6d
     t_, tref_;
   VectorUTheta  error_th_;
   MatrixRotation R_, Rref_, Rt_, Rreft_;
-  dg::Matrix P_, Pinv_;
+  Eigen::Matrix3d P_, Pinv_;
   double accuracy_;
   void inverseJacobianRodrigues ();
 } ;