ADD: new track message, Entity class and Position class

libs/eigen/doc/snippets/MatrixBase_noalias.cpp

@@ -0,0 +1,3 @@
+Matrix2d a, b, c; a << 1,2,3,4; b << 5,6,7,8;
+c.noalias() = a * b; // this computes the product directly to c
+cout << c << endl;