@Override
    public DataType setOutputType(DataType outputType, TypeContext context) {
        return super.setOutputType(DataType.STRING, outputType, null, context);
    }
Creates an executable ngram expression

@param linguistics the gram splitter to use, or null if this is used for representation and will not be executed
@param gramSize the gram size
indexinglanguage/src/main/java/com/yahoo/vespa/indexinglanguage/expressions/NGramExpression.java