Add a sample geoid grid from EIGEN-6C4

[leouieda]

Same configuration as the gravity and topography grid. Will be useful
when we need to convert heights or calculate water column corrections in
the oceans.

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.