More model manipulation functionalities

[yebai]

I find Soss's model manipulation functionalities quite nice, e.g., Do and predictive. We could implement some of these functionalities for BUGS models.

https://cscherrer.github.io/Soss.jl/stable/api/

cc @cscherrer

[cscherrer]

I hadn't heard of JuliaBUGS before, very nice!

[yebai]

We could have corresponding model operations for melding and cut as discussed for compositional inference.