Add reference ellipsoids for the Moon, Venus and Mercury

[santisoler]

Description of the desired feature

With the new Sphere class introduced in #42 for representing reference ellipsoids with zero flattening, now we are in position to add ellipsoids for planetary bodies that are usually treated as spheres, such as the Moon, Venus and Mercury.

@leouieda started adding the reference ellipsoid for the Moon on #13 before we had the Sphere class for that purpose, encountering the known problems of singularities due to division by zero (see #18).

The idea is to add new MOON, VENUS and MERCURY instances of the Sphere class with the correct parameters for the each body (they can be taken from Wieczorek (2015)). These three variables should be defined in the boule/realizations.py file.

Are you willing to help implement and maintain this feature?
Yes, I'm willing to help anyone that wants to implement it (yes, I'm talking to you)! 🚀

If you want to add just one of these bodies, it's ok: we can add each one of them on separated PRs.

[dabiged]

Hi All, first time contributor to boule.
I have had a go at adding VENUS. I am not sure about the syntax exactly in the declaration (sphere vs spheroid) or the doctest (should I test for semi-major axis and/or radius).

I am happy to take directions from more senior maintainers.

[leouieda]

@dabiged thanks for the PR 🙂 For Sphere, you could use either radius or semimajor_axis since it sets them both to the same value. Otherwise, all looks on and PR is merged (there was a slight mistake in the radius with a trailing 0 that shouldn't be there).

[leouieda]

That's what happens when I have 30 minutes to spare and don't want to open my emails 😉