ENH: support rhoRef for derived surfMesh sampled fields (issue #898)

tutorials/compressible/rhoSimpleFoam/squareBend/system/sampling

@@ -16,6 +16,18 @@ massflow
     fields          ( rhoU );
 }
 
+areaAverage
+{
+    ${__surfaceFieldValue}
+
+    regionType      surface;
+    name            plane1;
+
+    operation       weightedAreaAverage;
+    weightField     rhoU;
+    fields          ( p pTotal );
+}
+
 areaIntegrate
 {
     ${__surfaceFieldValue}