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_addition.cpp

@@ -0,0 +1,18 @@
+#include <Eigen/Dense>
+#include <iostream>
+
+using namespace Eigen;
+using namespace std;
+
+int main() {
+  ArrayXXf a(3, 3);
+  ArrayXXf b(3, 3);
+  a << 1, 2, 3, 4, 5, 6, 7, 8, 9;
+  b << 1, 2, 3, 1, 2, 3, 1, 2, 3;
+
+  // Adding two arrays
+  cout << "a + b = " << endl << a + b << endl << endl;
+
+  // Subtracting a scalar from an array
+  cout << "a - 2 = " << endl << a - 2 << endl;
+}