adds comment

cpp/full_code/pressure_stats.cpp

@@ -148,7 +148,13 @@ int pressure_stats<rnumber>::work_on_current_iteration(void)
     else
         stat_group = 0;
 
-    /// define local "compute scalar stats" function
+    /// define local "compute scalar stats" lambda function
+    /// it can't be a simple function, C++ doesn't allow that.
+    /// however, we want a simple function here because this whole block
+    /// consists of code that must be identical when applied to different
+    /// versions of the velocity field (i.e. vel field differently filtered).
+    /// luckily, C++11 allows lambda functions.
+    /// we must explicitly pass the "this" and "stat_group" names to the lambda function.
     auto compute_stats_for_cvelocity = [this, stat_group](std::string stat_prefix)
     {
         /// set scalar to 0