Move eigen to third party (#282)

* remove useless statement

* Add eigen to third_party dir

* remove reducdant lines

third_party/eigen/doc/snippets/Map_inner_stride.cpp

@@ -0,0 +1,7 @@
+int array[12];
+for (int i = 0; i < 12; ++i) array[i] = i;
+cout
+    << Map<VectorXi, 0, InnerStride<2> >(
+           array,
+           6)  // the inner stride has already been passed as template parameter
+    << endl;