Add ln prior to tractor outputs

[djbard]

Now that tractor can apply an interim prior, can the ln prior be reported along with the samples?

[davidwhogg]

This can be done by decorating the samples with blobs of meta-data, which is a functionality supported by emcee.

In general this could be also pull-requested at emcee because really with any MCMC code you are sampling a prior times a likelihood; each sample should then come with its prior and likelihood values attached (or the logs of those).

[dstndstn]

done, in MagicBeans / Reaper.