Add file_hash back to pooch.utils with a warning

[leouieda]

Moving this function to pooch.hashes caused crashes downstream. To prevent these crashes, add a wrapper back to utils that issues a warning to import from the top-level namespace instead.

Fixes #252

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.

[hmaarrfk]

Sorry for the trouble @leouieda !

[leouieda]

Not at all. Just glad we caught this quickly and the fix was relatively easy 🙂