Storm instantiation checkers for non-graph preserving assignments

[sjunges]

For future reference:

It would be very convenient if we could support instantiations checkers for non-graph preserving assignments; potentially via a flag that must be set explicitly and which leads to some overhead. Right now, the API for doing such a thing is a bit unclear (we can rebuild models, which is potentially a good alternative, but it would be nice if this could be opaque to the user).