public void addUserFieldSetItem(String setName, String field) {
        if (userFieldSets.get(setName) == null) {
            // First entry in this set
            userFieldSets.put(setName, new FieldSet(setName));
        }
        userFieldSets.get(setName).addFieldName(field);
    }
Adds an entry to user field sets, creating entries as needed

@param setName name of a field set
@param field field to add to field set
config-model/src/main/java/com/yahoo/schema/FieldSets.java