Move eigen to third party (#282)

* remove useless statement

* Add eigen to third_party dir

* remove reducdant lines

third_party/eigen/doc/examples/TutorialLinAlgInverseDeterminant.cpp

@@ -0,0 +1,13 @@
+#include <Eigen/Dense>
+#include <iostream>
+
+using namespace std;
+using namespace Eigen;
+
+int main() {
+  Matrix3f A;
+  A << 1, 2, 1, 2, 1, 0, -1, 1, 2;
+  cout << "Here is the matrix A:\n" << A << endl;
+  cout << "The determinant of A is " << A.determinant() << endl;
+  cout << "The inverse of A is:\n" << A.inverse() << endl;
+}