Move eigen to third party (#282)

* remove useless statement

* Add eigen to third_party dir

* remove reducdant lines

third_party/eigen/doc/snippets/FullPivLU_image.cpp

@@ -0,0 +1,9 @@
+Matrix3d m;
+m << 1, 1, 0, 1, 3, 2, 0, 1, 1;
+cout << "Here is the matrix m:" << endl << m << endl;
+cout << "Notice that the middle column is the sum of the two others, so the "
+     << "columns are linearly dependent." << endl;
+cout << "Here is a matrix whose columns have the same span but are linearly "
+        "independent:"
+     << endl
+     << m.fullPivLu().image(m) << endl;