Allow creation of InferenceData from CmdStanGQ objects

[WardBrian]

CmdStanPy allows you to run a standalone generated quantities run which returns a different data object than the standard .sample() method. That said, I believe it contains everything you'd need for a posterior-group-only InferenceData object.

This would be great for doing posterior predictive plotting - currently if I want to use arviz for this I need to re-run my whole model so I have a CmdStanMCMC object to pass to the arviz functions.

This was briefly mentioned in #844 but there appears to not be any more recent discussion

[ahartikainen]

I need to check my code, I think I have this implemented somewhere.

[WardBrian]

Thank you, it would be very helpful. I think I could rig something using some of the older code in the cmdstanpy io portion if you aren't able to find anything