Yields from models: tuning to match possible flow

[mahills]

Right now the yields from models are removing infeasible combinations of elements. These combinations may not be infeasible in certain conditions, though, such as when they are based on a conditional nested inside a loop. Then certain combinations are infeasible and certain are not. This needs to be modeled more precise.y.

Currently, to get all yields (including infeasible yields), call yields, but to filter (possibly missing some yields in certain cases) call yields with parameter filterYields set to true.