MTN: rehabilitate xpdSim

[CJ-Wright]

Currently our simulation logic is baked into the software itself, but we might try to separate the simulation and API. This way we can confirm that our software works on any system so long as it conforms to our simulation. Additionally this would help remove some pesky logic from the APIs which have corner cases for the simulation.

[sbillinge]

We should discuss this as a larger-scale refactoring effort towards the portable xpdAn. But good to keep this idea on the back burner for the Acq side too. As time goes on, the development intensity on the Acq side will decrease and I am less and less excited about putting the time into a shiny perfect simulation environment there....