Attach more metadata to distributions (e.g., sample size)

[longouyang]

Outlining some notes from an in-person conversation with Andreas:

• When doing viz.heatMap, we need to know how many samples there were -- it would be nice if this information was available in marginal distributions.
• It would be helpful to know if a Marginal distribution is supposed to represent a continuous or discrete posterior (this would be helpful for selecting an appropriate type in viz.auto).

[null-a]

Number of samples was added by #654.

[mhtess]

To add to this. In RWebPPL, the user can either pass in the inference algorithm options object from R, or just write it in the WebPPL program as we would normally do. When it's passed in from R, RWebPPL is able to attach the meta-data directly to the posterior object (useful, e.g., in the ggmcmc package). When inference is specified in WebPPL, however, we don't have access to the meta-data. Concretely, what we currently lack is:

• burn
• thin
• method

If this were accessible in the distributions object, that would be helpful