Remove the ellipsoid code in favour of Boule

[leouieda]

The ellipsoid code, together with the coordinate conversions and normal
gravity have been moved to Boule. Remove the associated code from
Harmonica and use Boule in the tests and examples that require it.

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.