functionObjects: Clear the result field from the objectRegistry if the argument fields are not available

This is needed to handle the processing of many time directories, some
of which may have the required fields and some not.

src/finiteVolume/fvMesh/fvMeshFunctionObject/fvMeshFunctionObject.H

@@ -95,6 +95,9 @@ protected:
         //- Write field if present in objectRegistry
         bool writeField(const word& fieldName);
 
+        //- Clear field from the objectRegistry if present
+        bool clearField(const word& fieldName);
+
 
 private: