make `Dataset.save` and `Dataset.load` methods work on directories as well as on TAR files

[xroynard]

Problem
The most used way of loading and saving a Dataset is with Dataset._save_to_dir_ and Dataset._load_from_dir_, but these methods have names starting with _

Solution
Improve methods load and save to work both on directories and archives, and avoid tempting the user to use methods starting with _

It should be done after #18 is merged

See #14 for retrocompatibility