January updates

We got awarded a Small Development Grant by NumFOCUS to validate poliastro against commercial and non-commercial similar applications! 🎉 Jorge started by setting GitHub Actions to trigger the validations automatically, validating our conversion between cartesian and Keplerian elements against Orekit, and is now fighting some discrepancies with Hohmann transfers.

On the development front, while we wait for Numba to release Python 3.9-compatible wheels for our next release, we refactored our Cowell propagator to make the code simpler and more numba-friendly, we tried to merge Eleftheria's implementation of the Escobal method for satellite visibility (and sadly got blocked in the process), and kept working on our analysis of Walker constellations for the OpenSatCom activity. We also engaged with the authors of the awesome numbakit-ode to rewrite our Cowell method using numba, but found some issues along the way and we decided to charge batteries and try again after some time.

And finally, we made a lot of documentation changes! Dhruv sent a series of PRs to amend some small issues, but most importantly he rewrote all the docs using MyST, a Markdown dialect compatible with Sphinx that is getting popular. We also removed the need to install LaTeX to compile our docs, and fixed some long standing problems with our Binder examples.