Restrict the function's return on autoformat
When the function started by NetArgumentParser returned e.g. a string, querying a json response worked but an xml response not. Vice versa, when the dictionary contained a numpy number, querying the xml response worked but the json response not, because the numpy number is not json serializable. Now, a **dictionary must be returned when autoformat is used** and the values of the dict must be either a nested dict, string or another non-dict/non-iterable that is convertible to str and json serializable. These rules apply to the nested dictionaries recursively.
Further, that what causes an error for the json conversion will also cause an error for the xml conversion and the other way round (better both response types work or do not work the same way than one response format works and the other not).