Write diagnostics with averaging

Author: joewallwork

core/src/Xios.cpp

@@ -950,6 +950,9 @@ void Xios::createField(const std::string fieldId)
     if (configGetInputRestartFieldNames().count(fieldId) > 0 || fieldId == maskName) {
         // Restarts are read "once" and the mask is fixed so only written once
         operation = "once";
+    } else if (configGetDiagnosticFieldNames().count(fieldId) > 0) {
+        // Diagonstics are averaged over the diagnostic output period
+        operation = "average";
     } else {
         // Otherwise, read/write all timesteps without post-processing
         operation = "instant";