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

[leouieda]

Fixed an issue in the CI (was setting the environment variable wrong). Will submit the same fix to Harmonica as well.