Move eigen to third party (#282)

* remove useless statement

* Add eigen to third_party dir

* remove reducdant lines

third_party/eigen/doc/examples/function_taking_eigenbase.cpp

@@ -0,0 +1,16 @@
+#include <Eigen/Core>
+#include <iostream>
+using namespace Eigen;
+
+template <typename Derived>
+void print_size(const EigenBase<Derived>& b) {
+  std::cout << "size (rows, cols): " << b.size() << " (" << b.rows() << ", "
+            << b.cols() << ")" << std::endl;
+}
+
+int main() {
+  Vector3f v;
+  print_size(v);
+  // v.asDiagonal() returns a 3x3 diagonal matrix pseudo-expression
+  print_size(v.asDiagonal());
+}