Feature/255 add option to load mednist in node

• dataset_manager: load_mednist_database; raise fedbiomedDatasetManagerError while loading mednist and loading images dataset
• cli: add new data_type mednist, with tags ['#MEDNIST', "#dataset"], name = 'MEDNIST' and description = 'MEDNIST dataset'
• exceptions: Implement FedbiomedDatasetManagerError
• constants: add new fedbiomed error constant FB315
• notebook: create new notebook pytorch-MEDNIST