Create list_container as container alternative for multiple objects with the same container path - #134

Temporary workaround for #45 without breaking container compatibility before version 2

SerializedFile's _container is also changed to be constructed using the list_container alternative so that all assets with a container path will properly show it instead of None when accessed with .container

A multiple_objects_per_container flag is introduced to ResourceManger.save() and extract_assets() in extractor.py to utilize list_container with a False default to avoid breaking compatibility