Move eigen to third party (#282)

* remove useless statement

* Add eigen to third_party dir

* remove reducdant lines

third_party/eigen/doc/examples/class_VectorBlock.cpp

@@ -0,0 +1,25 @@
+#include <Eigen/Core>
+#include <iostream>
+using namespace Eigen;
+using namespace std;
+
+template <typename Derived>
+Eigen::VectorBlock<Derived> segmentFromRange(MatrixBase<Derived>& v, int start,
+                                             int end) {
+  return Eigen::VectorBlock<Derived>(v.derived(), start, end - start);
+}
+
+template <typename Derived>
+const Eigen::VectorBlock<const Derived> segmentFromRange(
+    const MatrixBase<Derived>& v, int start, int end) {
+  return Eigen::VectorBlock<const Derived>(v.derived(), start, end - start);
+}
+
+int main(int, char**) {
+  Matrix<int, 1, 6> v;
+  v << 1, 2, 3, 4, 5, 6;
+  cout << segmentFromRange(2 * v, 2, 4) << endl;  // calls the const version
+  segmentFromRange(v, 1, 3) *= 5;                 // calls the non-const version
+  cout << "Now the vector v is:" << endl << v << endl;
+  return 0;
+}