ENH: emit "constant" for uniform constant field (#1269)

- this improves overall consistency and makes re-reading as a generic
  patch field behave properly when the underlying patch has zero size.

src/meshTools/PatchFunction1/ConstantField/ConstantField.C

@@ -253,9 +253,18 @@ void Foam::PatchFunction1Types::ConstantField<Type>::writeData
 ) const
 {
     PatchFunction1<Type>::writeData(os);
-    //os  << token::SPACE << value_ << token::END_STATEMENT << nl;
+
+    if (isUniform_)
+    {
+        os.writeKeyword(this->name_)
+            << "constant " << uniformValue_
+            << token::END_STATEMENT << nl;
+    }
+    else
+    {
         value_.writeEntry(this->name_, os);
     }
+}
 
 
 // ************************************************************************* //