Move eigen to third party (#282)

* remove useless statement

* Add eigen to third_party dir

* remove reducdant lines

third_party/eigen/doc/snippets/Tutorial_Map_using.cpp

@@ -0,0 +1,21 @@
+typedef Matrix<float, 1, Dynamic> MatrixType;
+typedef Map<MatrixType> MapType;
+typedef Map<const MatrixType> MapTypeConst;  // a read-only map
+const int n_dims = 5;
+
+MatrixType m1(n_dims), m2(n_dims);
+m1.setRandom();
+m2.setRandom();
+float *p = &m2(0);            // get the address storing the data for m2
+MapType m2map(p, m2.size());  // m2map shares data with m2
+MapTypeConst m2mapconst(p, m2.size());  // a read-only accessor for m2
+
+cout << "m1: " << m1 << endl;
+cout << "m2: " << m2 << endl;
+cout << "Squared euclidean distance: " << (m1 - m2).squaredNorm() << endl;
+cout << "Squared euclidean distance, using map: " << (m1 - m2map).squaredNorm()
+     << endl;
+m2map(3) = 7;  // this will change m2, since they share the same array
+cout << "Updated m2: " << m2 << endl;
+cout << "m2 coefficient 2, constant accessor: " << m2mapconst(2) << endl;
+/* m2mapconst(2) = 5; */  // this yields a compile-time error