Orekit validation first approach

[jorgepiloto]

This pull request is a first approach to poliastro's validation path. It makes use of the Orekit python wrapper and implements the following features:

• A class under the name OrekitOrbit, who's usage is similar to poliastro's Orbit class. Both of them try to model Keplerian orbits, which can be defined from vectors or classic orbital elements.

• A simple test regarding the conversion between RV and COE.

:warning: Orekit does not provide functions like rv2coe or coe2rv, since conversion is done directly in each of the available constructors of the KeplerianOrbit class in the Orekit library :warning:

Also notice the addition of the orekit-data.zip file, which holds necessary data for Orekit to properly work: ephemeris, time files... We might discuss if this data should be included within this repo or final user is required to download it from official channels.

[astrojuanlu]

We might discuss if this data should be included within this repo or final user is required to download it from official channels.

Let's not put it in the repo :pray: Otherwise it will make the whole git history too sluggish. If it's a common Orekit thing we can point to the relevant documentation, or even provide a script to download it.

I'll take a look at this tomorrow!

[jorgepiloto]

I just saw that the Orekit wrapper provides a download data function.

On the other hand, if the setup_orekit_dir() does not find the orekit-data.zip file in the local directory, en error message is shown but no exception is raised. This is weird when trying to manage program workflow...

[jorgepiloto]

Some updates on this PR regarding previous suggestions and feedback

I came up with the following structure:

orekit/
├── README.md
├── requirements.txt
├── src/
└── tests/

The src/ directory holds all logical files while the tests/ one is devoted to validation unit tests. In addition, a pytest.ini and conftest.py files are placed under this last folder, so anyone can run the tests following the same configuration.

Regarding orekit-data.zip:

Let's not put it in the repo pray Otherwise it will make the whole git history too sluggish. If it's a common Orekit thing we can point to the relevant documentation, or even provide a script to download it.

Now, there exists a setup_orekit_env() function under src/utils.py which checks if data is available in local machine. If not, it automatically downloads it from official sources and points orekit internals to data location, which I selected to be within src/data/.

[jorgepiloto]

Sorry, I just forgot to update the README.md installation section guide. I will solve for this this night.

[jorgepiloto]

I just added a propagate() method and its corresponding validation test, which passes without any problem. I think with all these features, we are ready to start building more complex tests such us maneuvers validation.

[astrojuanlu]

Merging! Thanks @jorgepiloto for taking this up and the patience 💪🏽

[astrojuanlu]

Oh, my bad... I now see that we merged orekit-data.zip here.

[astrojuanlu]

:warning: HISTORY OF THIS PULL REQUEST HAS BEEN REWRITTEN :warning: