Add argument to optionally keep offsets in InferenceData object returned by Model.fit()

[tomicapretto]

Offset variables were removed from the posterior in #276. However, it may be the case that someone may want to keep them.

This PR adds an argument keep_offsets to Model.fit() which defaults to False.

Checklist

• Add keep_offsets argument.
• Fix omit_vars documentation.
• Add tests.
• Discuss on existing argument names.

[tomicapretto]

Some thoughts on existing argument names. Currently we have

• omit_vars in Model.plot_priors(). This omits offsets and group specific terms.
• omit_offsets in Model.prior_predictive(). This omits offsets.

I am happy with omit_offsets because it clearly indicates its purpose. But I think omit_vars is ambiguous. Also, I think someone would like to plot group specific terms without having to plot offsets. I propose to replace omit_vars in Model.plot_priors() with

• omit_offsets: omit offsets.
• omit_group_specific: omit group specific effects.

What are your thoughts on this?

[aloctavodia]

I agree with split omit_var into two arguments. Also we should unify using "omit" or "keep". Both sound ok to me.