Small bug in bodies inheritance

[jorgepiloto]

🐞 Problem

While working with SSO orbits I discovered a bug 🐞

Earth.angular_velocity

Following error is raised:

AttributeError: '_Earth' object has no attribute 'angular_velocity'

This is because Earth inherits from _Earth and this one from _Body instead of Body, loosing last attributes introduced in #746 . Same error will be raised with other SS bodies.

📋 Steps to solve the problem

Inherit different private Solar System bodies classes from Body instead of _Body 🚀

[astrojuanlu]

This was fixed in #763.