[common] also count elementes entered into minmaxavg

dune/stuff/common/math.hh

@@ -188,6 +188,10 @@ public:
     acc_ = std::for_each(elements.begin(), elements.end(), acc_);
   }
 
+  std::size_t count() const
+  {
+    return boost::accumulators::count(acc_);
+  }
   ElementType sum() const
   {
     return boost::accumulators::sum(acc_);
@@ -218,7 +222,8 @@ public:
 
 protected:
   typedef boost::accumulators::stats<boost::accumulators::tag::max, boost::accumulators::tag::min,
-                                     boost::accumulators::tag::mean, boost::accumulators::tag::sum> StatsType;
+                                     boost::accumulators::tag::mean, boost::accumulators::tag::count,
+                                     boost::accumulators::tag::sum> StatsType;
   boost::accumulators::accumulator_set<ElementType, StatsType> acc_;
 };