Distance between points given in geodetic coordinates

[santisoler]

Add function to compute the Euclidean distance between two points given in geodetic coordinates.
The new function makes use of the closed-form formula given by Vajda (2004).
Add test function that compares the distance between two points, with the same distance computed by converting them to a spherical coordinate system and then using the distance_spherical function.

Fixes #154

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]

The function relies on the boule.Ellipsoid._prime_vertical_radius() method from fatiando/boule#35.
Until that PR is not merged and a new release of Boule available, CIs will fail.

[leouieda]

@santisoler thanks for getting this started! Left a few quick comments here.

[leouieda]

Some minor comments and this should be good to go once Boule 0.2 is out 👍