Add option to return normal gravity in SI units

[dabiged]

This PR adds an option to return gamma in SI units for the normal_gravity method in both the ellipsoid and sphere classes as per #46 . I have included an example in the sphere docstring and added tests for both classes.

Reminders:

• [x] Run make format and make check to make sure the code follows the style guide.
• [x] Add tests for new features or tests that would have caught the bug that you're fixing.
• [x] Add new public functions/methods/classes to doc/api/index.rst and the base __init__.py file for the package.
• [x] Write detailed docstrings for all functions/classes/methods. It often helps to design better code if you write the docstrings first.
• [x] 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.

[dabiged]

First time using pytest :blush: Thanks for the pointers. It is much cleaner now.

[leouieda]

First time using pytest

@dabiged it's pretty sweet, right? I only learned about these tricks several years after picking it up.

All that is left now is pleasing pylint. I left a comment above and that should do the trick.

Also, please feel free to add yourself to AUTHORS.md (if you'd like of course). We use that to give attribution when we publish Zenodo archives of new versions. If you add yourself, we'd be happy to list you as co-author on the archives 🙂 This is entirely optional, of course.

[leouieda]

👍🏽 merged! Loving all the PRs coming in @dabiged 🙂