Move eigen to third party (#282)

* remove useless statement

* Add eigen to third_party dir

* remove reducdant lines

third_party/eigen/doc/examples/class_CwiseBinaryOp.cpp

@@ -0,0 +1,20 @@
+#include <Eigen/Core>
+#include <iostream>
+using namespace Eigen;
+using namespace std;
+
+// define a custom template binary functor
+template <typename Scalar>
+struct MakeComplexOp {
+  EIGEN_EMPTY_STRUCT_CTOR(MakeComplexOp)
+  typedef complex<Scalar> result_type;
+  complex<Scalar> operator()(const Scalar& a, const Scalar& b) const {
+    return complex<Scalar>(a, b);
+  }
+};
+
+int main(int, char**) {
+  Matrix4d m1 = Matrix4d::Random(), m2 = Matrix4d::Random();
+  cout << m1.binaryExpr(m2, MakeComplexOp<double>()) << endl;
+  return 0;
+}