@Override
    public void write(ArithmeticValueUpdate update) {
        putInt(null, update.getOperator().id);
        putDouble(null, update.getOperand().doubleValue());
    }
Write out the value of weighted set field

@param field - field description (name and data type)
@param ws    - field value
document/src/main/java/com/yahoo/document/serialization/VespaDocumentSerializer6.java