Move eigen to third party (#282)

* remove useless statement

* Add eigen to third_party dir

* remove reducdant lines

third_party/eigen/doc/snippets/BiCGSTAB_step_by_step.cpp

@@ -0,0 +1,14 @@
+int n = 10000;
+VectorXd x(n), b(n);
+SparseMatrix<double> A(n, n);
+/* ... fill A and b ... */
+BiCGSTAB<SparseMatrix<double> > solver(A);
+// start from a random solution
+x = VectorXd::Random(n);
+solver.setMaxIterations(1);
+int i = 0;
+do {
+  x = solver.solveWithGuess(b, x);
+  std::cout << i << " : " << solver.error() << std::endl;
+  ++i;
+} while (solver.info() != Success && i < 100);