ADD: new track message, Entity class and Position class

libs/eigen/doc/snippets/MatrixBase_colwise.cpp

@@ -0,0 +1,5 @@
+Matrix3d m = Matrix3d::Random();
+cout << "Here is the matrix m:" << endl << m << endl;
+cout << "Here is the sum of each column:" << endl << m.colwise().sum() << endl;
+cout << "Here is the maximum absolute value of each column:"
+     << endl << m.cwiseAbs().colwise().maxCoeff() << endl;