ADD: new track message, Entity class and Position class

libs/eigen/doc/snippets/MatrixBase_eval.cpp

@@ -0,0 +1,12 @@
+Matrix2f M = Matrix2f::Random();
+Matrix2f m;
+m = M;
+cout << "Here is the matrix m:" << endl << m << endl;
+cout << "Now we want to copy a column into a row." << endl;
+cout << "If we do m.col(1) = m.row(0), then m becomes:" << endl;
+m.col(1) = m.row(0);
+cout << m << endl << "which is wrong!" << endl;
+cout << "Now let us instead do m.col(1) = m.row(0).eval(). Then m becomes" << endl;
+m = M;
+m.col(1) = m.row(0).eval();
+cout << m << endl << "which is right." << endl;