ADD: new track message, Entity class and Position class

libs/eigen/doc/snippets/MatrixBase_eigenvalues.cpp

@@ -0,0 +1,3 @@
+MatrixXd ones = MatrixXd::Ones(3,3);
+VectorXcd eivals = ones.eigenvalues();
+cout << "The eigenvalues of the 3x3 matrix of ones are:" << endl << eivals << endl;