add support for sum reduction to global communication

src/communication/mpi_global_policy.hpp

@@ -38,6 +38,11 @@ struct mpi_global_policy {
         return nest::mc::mpi::reduce(value, MPI_MAX);
     }
 
+    template <typename T>
+    static T sum(T value) {
+        return nest::mc::mpi::reduce(value, MPI_SUM);
+    }
+
     template <
         typename T,
         typename = typename std::enable_if<std::is_integral<T>::value>

src/communication/serial_global_policy.hpp

@@ -35,6 +35,11 @@ struct serial_global_policy {
         return value;
     }
 
+    template <typename T>
+    static T sum(T value) {
+        return value;
+    }
+
     template <
         typename T,
         typename = typename std::enable_if<std::is_integral<T>::value>