[common] adds sum to MinMaxAvg

dune/stuff/common/math.hh

@@ -162,6 +162,10 @@ public:
     acc_ = std::for_each(elements.begin(), elements.end(), acc_);
   }
 
+  ElementType sum() const
+  {
+    return boost::accumulators::sum(acc_);
+  }
   ElementType min() const
   {
     return boost::accumulators::min(acc_);
@@ -188,7 +192,7 @@ public:
 
 protected:
   typedef boost::accumulators::stats<boost::accumulators::tag::max, boost::accumulators::tag::min,
-                                     boost::accumulators::tag::mean> StatsType;
+                                     boost::accumulators::tag::mean, boost::accumulators::tag::sum> StatsType;
   boost::accumulators::accumulator_set<ElementType, StatsType> acc_;
 };