Add checks for ellipsoid parameters

[santisoler]

Add methods for validating ellipsoid parameters. Raise error if flattening is
equal to zero, but raise warning if it's lower than 1e-7. Raise error if
flattening is negative or greater than 1. Raise error if semimajor axis is not
positive. Raise warning if GM constant is negative. Add tests functions for new
features.

Fixes #43

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.

[santisoler]

As discussed on #42 , we should decide if we should raise an error when passing a zero flattening or not.
We agreed that if flattening is tiny we should not raise an error, but a warning instead.

[santisoler]

We have agreed to raise an error if flattening == 0, but only warn if flattening < 1e-7.