ADD: new track message, Entity class and Position class

libs/eigen/doc/snippets/FullPivLU_image.cpp

@@ -0,0 +1,9 @@
+Matrix3d m;
+m << 1,1,0,
+     1,3,2,
+     0,1,1;
+cout << "Here is the matrix m:" << endl << m << endl;
+cout << "Notice that the middle column is the sum of the two others, so the "
+     << "columns are linearly dependent." << endl;
+cout << "Here is a matrix whose columns have the same span but are linearly independent:"
+     << endl << m.fullPivLu().image(m) << endl;