Add pre-defined perturbation models

[astrojuanlu]

🎯 Goal

Whenever someone asks "can I analyze perturbations in poliastro?" my answer is: "yes, but you have to code them yourself". Most are non trivial and it would be nice to have some predefined models.

💡 Possible solutions

Perhaps there should be a factory to create an acceleration function from a set of parameters.

📋 Steps to solve the problem

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• Add, commit, push your changes
• Submit a pull request and add this in comments - Addresses #<put issue number here>
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[astrojuanlu]

Some of these perturbation models are sitting on an old computer of mine, waiting to be published:

https://groups.io/g/poliastro-dev/message/27

[astrojuanlu]

@nikita-astronaut is already taking care of this, see #341 and #343. We already did the required modifications to the propagation module and he will probably keep on adding some more. Closing!