ADD: new track message, Entity class and Position class

libs/eigen/doc/snippets/PartialRedux_sum.cpp

@@ -0,0 +1,3 @@
+Matrix3d m = Matrix3d::Random();
+cout << "Here is the matrix m:" << endl << m << endl;
+cout << "Here is the sum of each row:" << endl << m.rowwise().sum() << endl;