fix eigen deprecated warning (#432)

use eigen::all

Author: clarkliming

src/utils.h

@@ -29,7 +29,7 @@ T1 subset_matrix(T1 input, T2 index1, T2 index2) {
 template <typename T1, typename T2>
 T1 subset_matrix(T1 input, T2 index1) {
   #if EIGEN_VERSION_AT_LEAST(3,4,0)
-    T1 ret = input(index1, Eigen::placeholders::all);
+    T1 ret = input(index1, Eigen::all);
   #else
     T1 ret(index1.size(), input.cols());
     for (decltype(index1.size()) i = 0; i < index1.size(); i++) {