Move eigen to third party (#282)

* remove useless statement

* Add eigen to third_party dir

* remove reducdant lines

third_party/eigen/doc/snippets/Tutorial_AdvancedInitialization_LinSpaced.cpp

@@ -0,0 +1,7 @@
+ArrayXXf table(10, 4);
+table.col(0) = ArrayXf::LinSpaced(10, 0, 90);
+table.col(1) = M_PI / 180 * table.col(0);
+table.col(2) = table.col(1).sin();
+table.col(3) = table.col(1).cos();
+std::cout << "  Degrees   Radians      Sine    Cosine\n";
+std::cout << table << std::endl;