Forbid model publication method in read_only mode

You should not be able to use any of the current method of arkindex_worker.worker.training.TrainingMixin in read_only mode. This mixin is not supposed to be with an instance of ElementsWorker so the args attribute is not directly defined. However, in practice, the args are parsed at least to allow configuring the worker. Hence we can assume that read_only was computed before calling these methods.