ADD: new track message, Entity class and Position class

libs/eigen/doc/snippets/FullPivLU_solve.cpp

@@ -0,0 +1,11 @@
+Matrix<float,2,3> m = Matrix<float,2,3>::Random();
+Matrix2f y = Matrix2f::Random();
+cout << "Here is the matrix m:" << endl << m << endl;
+cout << "Here is the matrix y:" << endl << y << endl;
+Matrix<float,3,2> x = m.fullPivLu().solve(y);
+if((m*x).isApprox(y))
+{
+  cout << "Here is a solution x to the equation mx=y:" << endl << x << endl;
+}
+else
+  cout << "The equation mx=y does not have any solution." << endl;