Move wait() method from HistoryDatasetAssociation to Dataset superclass, so it can be applied to LibraryDatasets too.

Also move gi.histories.wait() to gi._wait_datasets() since it is not tied to histories (objects cleanup).
Deprecate create_user() method in favor of clearer create_remote_user(). Add create_local_user().

create_local_user() is also going to be used for Continuous Integration testing.
Don't fail the test if current_history is None.

Useful for Continuos Integration testing where the user is created via the API and does not have a session.