ADD: new track message, Entity class and Position class

libs/eigen/doc/snippets/TopicAliasing_mult4.cpp

@@ -0,0 +1,5 @@
+MatrixXf A(2,2), B(3,2);
+B << 2, 0,  0, 3, 1, 1;
+A << 2, 0, 0, -2;
+A = (B * A).cwiseAbs();
+cout << A;