added helper function for writing entries if not default

src/finiteVolume/fields/fvPatchFields/fvPatchField/fvPatchField.C

@@ -244,6 +244,23 @@ void Foam::fvPatchField<Type>::write(Ostream& os) const
 }
 
 
+template<class Type>
+template<class EntryType>
+void Foam::fvPatchField<Type>::writeEntryIfDifferent
+(
+    Ostream& os,
+    const word& entryName,
+    const EntryType& value1,
+    const EntryType& value2
+) const
+{
+    if (value1 != value2)
+    {
+        os.writeKeyword(entryName) << value2 << token::END_STATEMENT << nl;
+    }
+}
+
+
 // * * * * * * * * * * * * * * * Member Operators  * * * * * * * * * * * * * //
 
 template<class Type>

src/finiteVolume/fields/fvPatchFields/fvPatchField/fvPatchField.H

@@ -414,8 +414,21 @@ public:
             virtual void manipulateMatrix(fvMatrix<Type>& matrix);
 
 
-        //- Write
-        virtual void write(Ostream&) const;
+        // I-O
+
+            //- Write
+            virtual void write(Ostream&) const;
+
+            //- Helper function to write the keyword and entry only if the
+            //  values are not equal. The value is then output as value2
+            template<class EntryType>
+            void writeEntryIfDifferent
+            (
+                Ostream& os,
+                const word& entryName,
+                const EntryType& value1,
+                const EntryType& value2
+            ) const;
 
 
         // Check