ADD: new track message, Entity class and Position class

libs/eigen/doc/snippets/Tridiagonalization_householderCoefficients.cpp

@@ -0,0 +1,6 @@
+Matrix4d X = Matrix4d::Random(4,4);
+Matrix4d A = X + X.transpose();
+cout << "Here is a random symmetric 4x4 matrix:" << endl << A << endl;
+Tridiagonalization<Matrix4d> triOfA(A);
+Vector3d hc = triOfA.householderCoefficients();
+cout << "The vector of Householder coefficients is:" << endl << hc << endl;