add group info to summary

[ahartikainen]

Description

Add automatic group handling. Defaults to posterior, but works now with InferenceData without posterior (e.g. prior).

Checklist

• Follows official PR format
• Includes a sample plot to visually illustrate the changes (only for plot-related functions)
• New features are properly documented (with an example if appropriate)?
• Includes new or updated tests to cover the new feature
• Code style correct (follows pylint and black guidelines)
• Changes are listed in changelog

[ahartikainen]

I think this is ready.

Do you think 1 group is fine or do we want to enable multiple groups?

[OriolAbril]

LGTM, the comment is a minor nit that can be ignored