Move eigen to third party (#282)

* remove useless statement

* Add eigen to third_party dir

* remove reducdant lines

third_party/eigen/doc/snippets/compile_snippet.cpp.in

@@ -0,0 +1,23 @@
+static bool eigen_did_assert = false;
+#define eigen_assert(X) if(!eigen_did_assert && !(X)){ std::cout << "### Assertion raised in " << __FILE__ << ":" << __LINE__ << ":\n" #X << "\n### The following would happen without assertions:\n"; eigen_did_assert = true;}
+
+#include <iostream>
+#include <Eigen/Eigen>
+
+#ifndef M_PI
+#define M_PI 3.1415926535897932384626433832795
+#endif
+
+
+using namespace Eigen;
+using namespace std;
+
+int main(int, char**)
+{
+  cout.precision(3);
+// intentionally remove indentation of snippet
+{
+${snippet_source_code}
+}
+  return 0;
+}