Start unifying ellipsoids into a single class

[dabiged]

This PR unifies all ellipsoids (Spheres, oblate spheroids and triaxial ellipsoids) into a single class.

For discussion see #76

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 and the base __init__.py file for the package.
• [ ] Write detailed docstrings for all functions/classes/methods. It often helps to design better code if you write the docstrings first.
• [ ] If adding new functionality, add an example to the docstring, gallery, and/or tutorials.
• [ ] Add your full name, affiliation, and ORCID (optional) to the AUTHORS.md file (if you haven't already) in case you'd like to be listed as an author on the Zenodo archive of the next release.

[leouieda]

@dabiged thanks for starting this 🙂 I would suggest writing the tests first and then working on the implementation. We already have most of the tests for oblate ellipsoids and spheres and those should all pass with the new class.

For this PR, it might be best to omit anything for triaxial ellipsoids (raising NotImplementedError). This will already be a lot of work without having to figure out new tests for the triaxial part. We can then work on that in #72

I'm having a loot at what we can do with attrs to figure out the axis values from the flattening. I'll push some changes to this PR when I have a bit of time.

[leouieda]

@dabiged thanks for your work on this and sorry for letting this stall. I'm merging this into the dev branch as is so that it's registered in the history as your contributing. We'll continue to work on this in there until it's done and then merge into master.

Thanks again for all you've done in Boule and I hope we can restart this at some point 🙂

[dabiged]

Thanks for the update. I have a job now that isn't in the geophysics space so I will see when I can get back to this!

[leouieda]

@dabiged congrats on the new job 🎉 No rush and thanks for all you did already