ADD: new track message, Entity class and Position class

libs/eigen/doc/snippets/HouseholderQR_solve.cpp

@@ -0,0 +1,9 @@
+typedef Matrix<float,3,3> Matrix3x3;
+Matrix3x3 m = Matrix3x3::Random();
+Matrix3f y = Matrix3f::Random();
+cout << "Here is the matrix m:" << endl << m << endl;
+cout << "Here is the matrix y:" << endl << y << endl;
+Matrix3f x;
+x = m.householderQr().solve(y);
+assert(y.isApprox(m*x));
+cout << "Here is a solution x to the equation mx=y:" << endl << x << endl;