Improve and expand equinoctial elements handling

🐞 Problem

poliastro supports equinoctial elements, but test coverage is low and some conversions are not implemented.

🎯 Goal

Improve equinoctial element handling and showcase its advantages (no singularities, etc).

💡 Possible solutions

Some papers to start with:

• ~~Broucke & Cefola “On the equinoctial orbit elements” https://doi.org/10.1007/BF01228432~~
• ~~Cefola “Equinoctial orbit elements - Application to artificial satellite orbits” https://doi.org/10.2514/6.1972-937~~

📋 Steps to solve the problem

• Comment below about what you’ve started working on.
• Add, commit, push your changes
• Submit a pull request and add this in comments - Addresses #<put issue number here>
• Ask for a review in comments section of pull request
• Celebrate your contribution to this project 🎉

[jorgepiloto]

GMAT raises a singularity exception when the inclination of the orbit is 180, as the h and k elements are singular for that inclination value. Therefore, as suggested by @Yash-10, we should raise an exception in the TODO line, see:

https://github.com/poliastro/poliastro/blob/bebbf617e0ebfb797ba1c593b8241e4cf0f9c466/src/poliastro/core/elements.py#L226-L228

[astrojuanlu]

@jorgepiloto saw that at least the latest GMAT version ships with Modified Equinoctial Elements, so perhaps now we can validate them!