Generalized recursive networks

[jcampolongo]

Currently we have Expanders, but they were an early development in Scruff and are clunky. It would be better if variables could be associated with NetworkModels, which generate NetworkSFuncs.

Something like this exists already - need to check if it is complete and works as expected, and document it.
Then, if we have higher order SFuncs over NetworkSFuncs, they will subsume Expanders.

Finally, we need to make LazyInference work with any embedded Network/NetworkSFuncs and not just Expanders.
Once all this is done, we can deprecate Expanders.