Refactor plotting to use backends and a consistent notebook experience

🐞 Problem

The experience with 2D and 3D plots in the notebook is inconsistent, due to the inconsistencies in the rendering between matplotlib and plotly. The transition from Jupyter notebook to JupyterLab is making things more difficult, as the documentation is scattered and now the users need to install JavaScript extensions as well.

Also, OrbitPlotter and OrbitPlotter3D do not share a common parent class and API, which is also a problem since most of the functionality is shared.

Related: #315 and #316.

🎯 Goal

1. Refactor the plotting module so it allows the user to configure a “backend”, via an environment variable or an IPython magic
2. Unify the OrbitPlotter and OrbitPlotter3D classes, putting common functionality in a parent superclass
3. Use Plotly for the notebook, both in 2D and 3D, and leave a matplotlib backend for those that want static, publication-quality 2D

💡 Possible solutions

📋 Steps to solve the problem

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[astrojuanlu]

for the moment yes, although BaseOrbitPlotter should perhaps be private (so _BaseOrbitPlotter) and we might think of just having one class and switching the behavior using a flag, or something like that

[astrojuanlu]

Now that OrbitPlotter2D shows nice plots, it’s time to start unifying OrbitPlotter2D and OrbitPlotter3D finding common code and putting it in a base class. I think we can take care of OrbitPlotter separately.