Add geodetic to geocentric coordinates conversion

[santisoler]

Convert geocentric coordinates (defined by the ReferenceEllipsoid) to
geocentric (spherical) coordinates. Only geodetic latitude and height
are converted to geocentric_latitude and radius, geocentric and
geodetic latitudes are equal. Add tests for new function: (1) test with
a "spherical" ellipsoid (in such case the geodetic and gecentric
coordinates are the same) (2) test with real ellipsoids on equator and
poles (on those points geodetic and geocentric latitudes are must be
equal).

Fixes #20

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.

[santisoler]

@leouieda
The CI will fail because ReferenceEllipsoid does not have first_eccentricty among its attributes on this branch.
We should merge #27 before this.

[leouieda]

👍 Merged #27 so feel free to update/rebase this branch.

[leouieda]

@santis19 just a minor comment but something we should think about. We can also merge this and change the name later but that's usually more work.

[leouieda]

Nice! I'm increasingly happy with the ellipsoid mechanics in Harmonica.

[leouieda]

I merged in the changes from master so please remember to git pull

[leouieda]

@santis19 this looks good to me.

Just thinking if we should standardize the API for things that take in coordinates. It might be good to always have (longitude, latitude, height) so that we make calls like geodetic_to_spherical(*coordinates) instead of geodetic_to_spherical(coordinates[1], coordinates[2]). What do think?

[leouieda]

I'm going to merge this and we'll think about the arguments a bit more before making changes.

Thanks @santis19