Move eigen to third party (#282)

* remove useless statement

* Add eigen to third_party dir

* remove reducdant lines

third_party/eigen/doc/snippets/LLT_solve.cpp

@@ -0,0 +1,10 @@
+typedef Matrix<float, Dynamic, 2> DataMatrix;
+// let's generate some samples on the 3D plane of equation z = 2x+3y (with some
+// noise)
+DataMatrix samples = DataMatrix::Random(12, 2);
+VectorXf elevations =
+    2 * samples.col(0) + 3 * samples.col(1) + VectorXf::Random(12) * 0.1;
+// and let's solve samples * [x y]^T = elevations in least square sense:
+Matrix<float, 2, 1> xy =
+    (samples.adjoint() * samples).llt().solve((samples.adjoint() * elevations));
+cout << xy << endl;