Move eigen to third party (#282)

* remove useless statement

* Add eigen to third_party dir

* remove reducdant lines

third_party/eigen/doc/snippets/FullPivLU_kernel.cpp

@@ -0,0 +1,8 @@
+MatrixXf m = MatrixXf::Random(3, 5);
+cout << "Here is the matrix m:" << endl << m << endl;
+MatrixXf ker = m.fullPivLu().kernel();
+cout << "Here is a matrix whose columns form a basis of the kernel of m:"
+     << endl
+     << ker << endl;
+cout << "By definition of the kernel, m*ker is zero:" << endl
+     << m * ker << endl;