ADD: new track message, Entity class and Position class

libs/eigen/doc/snippets/ComplexEigenSolver_eigenvalues.cpp

@@ -0,0 +1,4 @@
+MatrixXcf ones = MatrixXcf::Ones(3,3);
+ComplexEigenSolver<MatrixXcf> ces(ones, /* computeEigenvectors = */ false);
+cout << "The eigenvalues of the 3x3 matrix of ones are:" 
+     << endl << ces.eigenvalues() << endl;