Replace 'DirectoryEntry.ls()' method by 'files' and 'dirs' properties

and make the retrieval of children from the archive explicit in a
dedicated retrieve_children() method.

Change the API of ProvenanceInteface's content_get_early_dates() and
directory_get_dates_in_isochrone_frontier to expect Iterable instead of
List to prevent having to create unneeded temporary lists from
generators retrieved from 'Directory.files' and '.dirs'.