Fix bug when unpacking a subfolder from an archive

[leouieda]

Our unpacking processors (Untar and Unzip) aren't handling members that are a whole subfolder correctly. The elements of members are being treated as files regardless of what they are. Add a test for use case (no change to our test data required) and refactor the unpacking test to make it easier to extend in the future.

Fixes #264

Reminders:

• Run make format and make check to make sure the code follows the style guide.
• Add tests for new features or tests that would have caught the bug that you're fixing.
• Add new public functions/methods/classes to doc/api/index.rst and the base __init__.py file for the package.
• Write detailed docstrings for all functions/classes/methods. It often helps to design better code if you write the docstrings first.
• If adding new functionality, add an example to the docstring, gallery, and/or tutorials.
• Add your full name, affiliation, and ORCID (optional) to the AUTHORS.md file (if you haven't already) in case you'd like to be listed as an author on the Zenodo archive of the next release.

[leouieda]

@adam2392 this should fix the issue. Would you be able to try using the version in the unpack-bug branch to see if it works for you?

[leouieda]

Thanks for the review @hugovk 👍🏽 Merging this and making a new release. Hopefully nothing will break upstream this time 🤞🏽