Add a warning if a point is outside the ellipsoid

[MGomezN]

Add warning for the normal gravity formula when the height is negative Issue #44

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.

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[santisoler]

Congratulations @MGomezN for opening the PR! :tada: :medal_sports: :rocket:

Let me know if you need some code review or any help!

[leouieda]

@MGomezN thanks for the contribution! It's much appreciated!

One thing I noticed is that your commits don't seem to be linked to your GitHub account (your profile picture should appear instead of the gray GitHub icons). This usually happens when you use a different email to configure git on your machine than the one you used with your GitHub account. My recommendation would be to configure git on your machine with:

1. You full name: `git config --global user.name "Leonardo Uieda"
2. The email you use for GitHub: `git config --global user.email "leouieda@github.com"

See http://swcarpentry.github.io/git-novice/02-setup/index.html

You don't need to change the existing commits (that will be fixed when we merge this PR) but you might want to do that for future commits.

[MGomezN]

Congratulations @MGomezN for opening the PR! 🎉 🏅 🚀

Let me know if you need some code review or any help!

Thank you! Yes! I couldn't test my modifications, but I think I know what the problem is... I will try to work on Fridays, Saturdays, and Sundays. So, let me try to fix as much as I can and I will come back to you for help. Thank you!

[MGomezN]

@MGomezN thanks for the contribution! It's much appreciated!

One thing I noticed is that your commits don't seem to be linked to your GitHub account (your profile picture should appear instead of the gray GitHub icons). This usually happens when you use a different email to configure git on your machine than the one you used with your GitHub account. My recommendation would be to configure git on your machine with:

1. Your full name: `git config --global user.name "Leonardo Uieda"
2. The email you use for GitHub: `git config --global user.email "leouieda@github.com"

See http://swcarpentry.github.io/git-novice/02-setup/index.html

You don't need to change the existing commits (that will be fixed when we merge this PR) but you might want to do that for future commits.

Thank you @leouieda, as I said in the slack channel, I think I will cause too much trouble in the beginning. Thank you for the patience