WIP Add an ellipsoid for the Moon

[leouieda]

Use values from the compilation in Wieczorek (2015).

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]

Tests fail because the equations fail (nan) when the flattening is 0. That leads to eccentricities being 0 and there are divisions of these values in the equations. The only sensible way forward I can see is to create a separate Sphere class that has simplified equations for a reference sphere. It would actually be a lot simpler since we'd be using spherical coordinates by default and so wouldn't need the coordinate conversions. The gravity equations is also a lot simpler.

[leouieda]

Maybe a better way forward is to check if the flattening is close to zero and then use the sphere equation instead. That way we don’t need a new class and the associated maintenance.

[santisoler]

I have updated this PR by defining the MOON ellipsoids as an instance of the new Sphere class introduced on #42.
I faced a bug on how the Sphere class computes the gravity on the equator, which should be fixed on #48.
After that, I think the Moon implementation should be ready, maybe we could add a gallery example for it.

[leouieda]

Closing this since the approach wouldn't work. See #53 instead.