Add property to remember the list of free RVs

[michaelosthege]

Tell us about it

By passing var_names = [rv.name for rv in pmodel.free_RVs] I can limit, for example, plot_trace to exclude deterministic variables, but it needs the model object.
After loading an InferenceData from a file this information is no longer around.

Thoughts on implementation

An attribute such as idata.posterior.free_RVs could be attached and used by some ArviZ plots as the default var_names.

[OriolAbril]

xarray datasets allow storing any attributes in the dataset.attrs dict, where we store the versions, time... I think ideally this would go there.

There is one caveat however which is that lists may not be allowed as attrs by netcdf, so we should probably extend to/from_netcdf to optionally convert/parse lists/dicts to/from strings using json for example. I think that would also be useful for some custom stan attributes like init point or mass matrix that IIRC are currently stored as string attrs.

[michaelosthege]

Oh should I have opened the issue in PyMC3, now that the converter lives there?
But the idea with the JSON also sounds like something that could generalize - just giving the converters a way to add custom fields.

[ahartikainen]

What is a free_RV? Could we use similar logic for sparse datastructures from Stan side (e.g. matrices with half containing nan -> cholesky)

[michaelosthege]

In PyMC3 terminology the free RVs are the input variables to the model. The sampler is concerned with these variables, and while deterministics are also part of the MCMC trace, they are often not interesting to look at in a traceplot.

[OriolAbril]

related to #420