Use a decompression processor for ETOPO1

[leouieda]

Create a Pooch processor that decompresses the ETOPO1 grid so that we can load it in xarray without having to load everything into memory.

Fixes #20

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.