Add option to disable updates

[dopplershift]

This gives users the option, when creating a Pooch, to disallow updating local copies of files. The intended use case for this is for development checkouts/CI systems where the local copies are canonical and any need to update likely reflects a problem with the registry (like and out of date hash).

Fixes #290.

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.

[welcome]

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• Authorship Guidelines: Our rules for giving you credit for your contributions, including authorship on publications and Zenodo archives.
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[leouieda]

👍🏽 looks great @dopplershift! Merging this in. I'll make a new release as soon as I can.

[welcome]

🎉 Congrats on merging your first pull request and welcome to the team! 🎉

If you would like to be added as a author on the Zenodo archive of the next release, add your full name, affiliation, and ORCID (optional) to the AUTHORS.md file of this repository. Feel free to do this in a new pull request if needed.

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