ADD: new track message, Entity class and Position class

libs/eigen/doc/snippets/BiCGSTAB_simple.cpp

@@ -0,0 +1,11 @@
+  int n = 10000;
+  VectorXd x(n), b(n);
+  SparseMatrix<double> A(n,n);
+  /* ... fill A and b ... */ 
+  BiCGSTAB<SparseMatrix<double> > solver;
+  solver.compute(A);
+  x = solver.solve(b);
+  std::cout << "#iterations:     " << solver.iterations() << std::endl;
+  std::cout << "estimated error: " << solver.error()      << std::endl;
+  /* ... update b ... */
+  x = solver.solve(b); // solve again