Move eigen to third party (#282)

* remove useless statement

* Add eigen to third_party dir

* remove reducdant lines

third_party/eigen/doc/snippets/MatrixBase_computeInverseWithCheck.cpp

@@ -0,0 +1,10 @@
+Matrix3d m = Matrix3d::Random();
+cout << "Here is the matrix m:" << endl << m << endl;
+Matrix3d inverse;
+bool invertible;
+m.computeInverseWithCheck(inverse, invertible);
+if (invertible) {
+  cout << "It is invertible, and its inverse is:" << endl << inverse << endl;
+} else {
+  cout << "It is not invertible." << endl;
+}