ADD: new track message, Entity class and Position class

libs/eigen/doc/examples/class_FixedBlock.cpp

@@ -0,0 +1,27 @@
+#include <Eigen/Core>
+#include <iostream>
+using namespace Eigen;
+using namespace std;
+
+template<typename Derived>
+Eigen::Block<Derived, 2, 2>
+topLeft2x2Corner(MatrixBase<Derived>& m)
+{
+  return Eigen::Block<Derived, 2, 2>(m.derived(), 0, 0);
+}
+
+template<typename Derived>
+const Eigen::Block<const Derived, 2, 2>
+topLeft2x2Corner(const MatrixBase<Derived>& m)
+{
+  return Eigen::Block<const Derived, 2, 2>(m.derived(), 0, 0);
+}
+
+int main(int, char**)
+{
+  Matrix3d m = Matrix3d::Identity();
+  cout << topLeft2x2Corner(4*m) << endl; // calls the const version
+  topLeft2x2Corner(m) *= 2;              // calls the non-const version
+  cout << "Now the matrix m is:" << endl << m << endl;
+  return 0;
+}