Move eigen to third party (#282)

* remove useless statement

* Add eigen to third_party dir

* remove reducdant lines

third_party/eigen/doc/examples/Tutorial_ArrayClass_mult.cpp

@@ -0,0 +1,13 @@
+#include <Eigen/Dense>
+#include <iostream>
+
+using namespace Eigen;
+using namespace std;
+
+int main() {
+  ArrayXXf a(2, 2);
+  ArrayXXf b(2, 2);
+  a << 1, 2, 3, 4;
+  b << 5, 6, 7, 8;
+  cout << "a * b = " << endl << a * b << endl;
+}