very rough draft multi-chain code

[alanocallaghan]

This was one of the review suggestions, I think it's a good idea.

Probably will still want to:

• [ ] add functionality to collapse the multiple draws to a single chain, or
• [ ] make the multichain object have a similar interface to normal chains by wrapping the extraction of draws

and

• [ ] Implement some kind of Rhat guidance for chains
• [ ] improve the multichain showfit with smoothing/binning etc
• [ ] document and add some basic tests