Deprecate the use of from_pymc3 outside the model context

[AlexAndorra]

When people give PyMC's MultiTrace objects to ArviZ functions outside the model context, this raises a FutureWarning because it calls az.from_pymc3 under the hood, which is supposed to stop working without a model context.

As this warning has been there for a while now, we should actually go ahead and deprecate this possibility, especially as PyMC is going to switch to returning InferenceData by default in 4.0.0.

Feel free to comment if you want to take on this issue 🖖

[OriolAbril]

Closing this as we moved the converter to pymc3 codebase and the model context is already required in the v4 version