ADD: new track message, Entity class and Position class

libs/eigen/doc/snippets/Tridiagonalization_packedMatrix.cpp

@@ -0,0 +1,8 @@
+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);
+Matrix4d pm = triOfA.packedMatrix();
+cout << "The packed matrix M is:" << endl << pm << endl;
+cout << "The diagonal and subdiagonal corresponds to the matrix T, which is:" 
+     << endl << triOfA.matrixT() << endl;