ADD: new track message, Entity class and Position class

libs/eigen/doc/snippets/MatrixBase_computeInverseAndDetWithCheck.cpp

@@ -0,0 +1,13 @@
+Matrix3d m = Matrix3d::Random();
+cout << "Here is the matrix m:" << endl << m << endl;
+Matrix3d inverse;
+bool invertible;
+double determinant;
+m.computeInverseAndDetWithCheck(inverse,determinant,invertible);
+cout << "Its determinant is " << determinant << endl;
+if(invertible) {
+  cout << "It is invertible, and its inverse is:" << endl << inverse << endl;
+}
+else {
+  cout << "It is not invertible." << endl;
+}