Remove private prefix of `Archive._get_entry_by_id`

`Archive._get_entry_by_id()` has been there, prefixed, since the beginning of the wrapper.

We now has enough experience to say that it serves some use cases that are not served by other means, including #94.
It's true that it is not common to use it, but so are many parts of the API.

We should thus remove the prefix (and keep a deprecated alias until next major)

See #224 