Move eigen to third party (#282)

* remove useless statement

* Add eigen to third_party dir

* remove reducdant lines

third_party/eigen/doc/examples/Tutorial_BlockOperations_colrow.cpp

@@ -0,0 +1,15 @@
+#include <Eigen/Dense>
+#include <iostream>
+
+using namespace std;
+
+int main() {
+  Eigen::MatrixXf m(3, 3);
+  m << 1, 2, 3, 4, 5, 6, 7, 8, 9;
+  cout << "Here is the matrix m:" << endl << m << endl;
+  cout << "2nd Row: " << m.row(1) << endl;
+  m.col(2) += 3 * m.col(0);
+  cout << "After adding 3 times the first column into the third column, the "
+          "matrix m is:\n";
+  cout << m << endl;
+}