Add the age of the oceanic lithosphere grids

[leouieda]

Fetch and load the netCDF version of the age and uncertainty grids from the NOAA NCEI website. Available in the 2 and 6 arc-minute resolutions. Add metadata to the Dataset, like units, long names, and the Müller et al (2008) paper DOI.

Fixes #9

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]

@santisoler I think we should make a release after this gets merged. What do you think? We can add datasets slowly and make new releases whenever we add a couple. This way we can start using and promoting the package.

[santisoler]

I agree @leouieda. From the experience with Verde and Pooch, we get a lot of feedback after a library is released. So I think it's better to make frequent releases with new changes rather than expecting to have big changes. This way it get publicity and attention, bringing new potential users and more feedback.