ADD: new track message, Entity class and Position class

libs/eigen/doc/snippets/SelfAdjointEigenSolver_SelfAdjointEigenSolver.cpp

@@ -0,0 +1,7 @@
+SelfAdjointEigenSolver<Matrix4f> es;
+Matrix4f X = Matrix4f::Random(4,4);
+Matrix4f A = X + X.transpose();
+es.compute(A);
+cout << "The eigenvalues of A are: " << es.eigenvalues().transpose() << endl;
+es.compute(A + Matrix4f::Identity(4,4)); // re-use es to compute eigenvalues of A+I
+cout << "The eigenvalues of A+I are: " << es.eigenvalues().transpose() << endl;