Pathfinder should return simulatable structure

[jmorris335]

In addition to performing the simulation, the pathfinder should also return a simulatable structure, something that can simulate different inputs as well. It's not always possible for these inputs to be simulated--not every input will be in the program's validity frame--but at least there should be a way to try repeated calls. This should probably be it's own class, Simulatable or something similar, that raises exceptions when inputs are passed that violate a model's validity frame (the via method).