Use pooch.os_cache and pkg_resources in datasets

[leouieda]

With Pooch 0.7.0, the recommended way of loading the registry file is
with pkg_resources (see fatiando/pooch#120). It's also better to use
the default cache location so users can more easily clean up unused
files. Because this is system specific, add the
harmonica.datasets.locate function to return the cache folder
location.

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.
• Write detailed docstrings for all functions/methods.
• If adding new functionality, add an example to the docstring, gallery, and/or tutorials.

[leouieda]

Same PR as in Verde. Feel free to merge when/if tests pass :)