Move eigen to third party (#282)

* remove useless statement

* Add eigen to third_party dir

* remove reducdant lines

third_party/eigen/doc/examples/TutorialLinAlgRankRevealing.cpp

@@ -0,0 +1,19 @@
+#include <Eigen/Dense>
+#include <iostream>
+
+using namespace std;
+using namespace Eigen;
+
+int main() {
+  Matrix3f A;
+  A << 1, 2, 5, 2, 1, 4, 3, 0, 3;
+  cout << "Here is the matrix A:\n" << A << endl;
+  FullPivLU<Matrix3f> lu_decomp(A);
+  cout << "The rank of A is " << lu_decomp.rank() << endl;
+  cout
+      << "Here is a matrix whose columns form a basis of the null-space of A:\n"
+      << lu_decomp.kernel() << endl;
+  cout << "Here is a matrix whose columns form a basis of the column-space of "
+          "A:\n"
+       << lu_decomp.image(A) << endl;  // yes, have to pass the original A
+}