Raise informative error if dictionary is passed as observed of DensityDist

[ricardoV94]

This is a common source of confusion in people switching from V3 to V4. We could print an informative message when users try to do it.

[juanitorduz]

It is still a confusion for me (I had to use pm.Potential) 🙈 , what is the correct type for the observed parameter? (I can add a fix accordingly)

[ricardoV94]

observed should be exactly the same as for other distributions (pd.Series, np.array, pm.Data). Dictionaries are no longer a thing.

[michaelosthege]

Closed by #6292