ADD: new track message, Entity class and Position class

libs/eigen/doc/snippets/Tridiagonalization_compute.cpp

@@ -0,0 +1,9 @@
+Tridiagonalization<MatrixXf> tri;
+MatrixXf X = MatrixXf::Random(4,4);
+MatrixXf A = X + X.transpose();
+tri.compute(A);
+cout << "The matrix T in the tridiagonal decomposition of A is: " << endl;
+cout << tri.matrixT() << endl;
+tri.compute(2*A); // re-use tri to compute eigenvalues of 2A
+cout << "The matrix T in the tridiagonal decomposition of 2A is: " << endl;
+cout << tri.matrixT() << endl;