User-registered model (#391)

Add a jupyter notebook showing how to register a user's (external) image-classification model.
Contains fixes to the previous models extension mechanism, and relaxation of the `args' requirements in apputils/image_classifier.py.

apputils/image_classifier.py –
*when self.logdir is None:
-use NullLogger
-skip save_checkpoint

*return training log from run_training_loop()
*don’t log if script_dir or output_dir are not set.
*Fix params_nnz_cnt in update_training_scores_history()

data_loggers/logger.py – add NullLogger which does not log