codedFunctionObjectTemplate: Added #include "volFields.H"

The codedFunctionObjectTemplate is based on regionFunctionObject requiring
fvMesh.H and most manipulate volFields so it makes sense for volFields.H to be
included by default.

tutorials/modules/fluid/blockedChannel/system/generateAlphas

@@ -7,6 +7,8 @@
 \*---------------------------------------------------------------------------*/
 
 type            coded;
+
+// Load the library containing the 'coded' functionObject
 libs            ("libutilityFunctionObjects.so");
 
 name            generateAlphas;