add benchmark for sum

bench/bench_sum.cpp

@@ -0,0 +1,17 @@
+#include <Eigen/Core>
+USING_PART_OF_NAMESPACE_EIGEN
+using namespace std;
+
+int main() 
+{
+  typedef Matrix<SCALAR,Eigen::Dynamic,1> Vec;
+  Vec v(SIZE);
+  v.setZero();
+  v[0] = 1;
+  v[1] = 2;
+  for(int i = 0; i < 1000000; i++)
+  {
+    v.coeffRef(0) += v.sum() * SCALAR(1e-20);
+  }
+  cout << v.sum() << endl;
+}