Conditional relationships in dynamic models

[christophergandrud]

I forgot to add implementing Zach Warner's approach to conditional relationships for dynamic models under our time series goals.

I actually had thrown together a proto-package for this in the fall: https://github.com/christophergandrud/ecmSim

A more general point for selling Zelig seems to be playing up its usefulness for showing conditional relationships. This is why I created simPH, for example.