public void writeFunction(String name, RankingExpression expression) {
            application.getFile(modelFiles.functionsPath()).appendFile(name + "\t" +
                                                                       expression.getRoot().toString() + "\n");
        }
Adds this function expression to the application package so it can be read later.
config-model/src/main/java/com/yahoo/vespa/model/ml/ConvertedModel.java