Add other data to schema?

Per #219, other data may be involved in inference. In the eight schools model, for example, the data block contains:

data {
  int<lower=0> J; // number of schools 
  real y[J]; // estimated treatment effects
  real<lower=0> sigma[J]; // s.e. of effect estimates 
}

right now, we only extract “observed_data”, which are observations of parameters in the model. More specifically, the posterior_predictive/ group should match with the observed_data/ group. Open for suggestions on whether and how to store the rest of the data.

[ahartikainen]

For merging, let’s just decide a good way to do it (e.g. iloc 0 to 1000 … pystan infom 1000 to 2000 pymc info if combining pystan + pymc. Then just keep on updating this depending how we concat. Slicing adds some extra complexity)

[OriolAbril]

I want to leave a link to the current specification on InferenceData schema here: https://arviz-devs.github.io/arviz/schema/schema.html

Moreover, I realized that @tyarkoni will probably have useful insight on sampler parameters/backend parameters and the like. Anything you would like to share will be more than welcome.