Use the new pooch.Decompress on ETOPO1 and seafloor

[santisoler]

Replace the Decompress classes with the new pooch.Decompress.

The minimum required version of Pooch was raised to v0.5.0. Update tests for uncompressed filenames, they will be in the form of {file}.decomp.

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.