Automatically add trailing slash to base_url on pooch.create

[gabrielfu]

Automatically add a trailing "/" to base_url in pooch.create() if missing. Update docstring and add test function that checks if the trailing slash gets added.

Relevant issues/PRs:

Fixes #324

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[santisoler]

Thanks for the contribution @gabrielfu! I think this is ready to be merged! I just added a small test function to prevent us breaking this feature in the future.

I'm merging it as soon as the Actions pass. 🚀

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