Move eigen to third party (#282)

* remove useless statement

* Add eigen to third_party dir

* remove reducdant lines

third_party/eigen/doc/snippets/TopicAliasing_cwise.cpp

@@ -0,0 +1,18 @@
+MatrixXf mat(2, 2);
+mat << 1, 2, 4, 7;
+cout << "Here is the matrix mat:\n" << mat << endl << endl;
+
+mat = 2 * mat;
+cout << "After 'mat = 2 * mat', mat = \n" << mat << endl << endl;
+
+mat = mat - MatrixXf::Identity(2, 2);
+cout << "After the subtraction, it becomes\n" << mat << endl << endl;
+
+ArrayXXf arr = mat;
+arr = arr.square();
+cout << "After squaring, it becomes\n" << arr << endl << endl;
+
+// Combining all operations in one statement:
+mat << 1, 2, 4, 7;
+mat = (2 * mat - MatrixXf::Identity(2, 2)).array().square();
+cout << "Doing everything at once yields\n" << mat << endl << endl;