Move eigen to third party (#282)

* remove useless statement

* Add eigen to third_party dir

* remove reducdant lines

third_party/eigen/doc/snippets/SparseMatrix_coeffs.cpp

@@ -0,0 +1,13 @@
+SparseMatrix<double> A(3, 3);
+A.insert(1, 2) = 0;
+A.insert(0, 1) = 1;
+A.insert(2, 0) = 2;
+A.makeCompressed();
+cout << "The matrix A is:" << endl << MatrixXd(A) << endl;
+cout << "it has " << A.nonZeros()
+     << " stored non zero coefficients that are: " << A.coeffs().transpose()
+     << endl;
+A.coeffs() += 10;
+cout << "After adding 10 to every stored non zero coefficient, the matrix A is:"
+     << endl
+     << MatrixXd(A) << endl;