void writeExpression(String name, ExpressionFunction expression) {
            StringBuilder b = new StringBuilder(expression.getBody().getRoot().toString());
            for (Map.Entry<String, TensorType> input : expression.argumentTypes().entrySet())
                b.append('\n').append(input.getKey()).append('\t').append(input.getValue());
            application.getFile(modelFiles.expressionPath(name)).writeFile(new StringReader(b.toString()));
        }
Adds this expression to the application package, such that it can be read later.

@param name the name of this ranking expression - may have 1-3 parts separated by dot where the first part
            is always the model name
config-model/src/main/java/com/yahoo/vespa/model/ml/ConvertedModel.java