Enable recomputing y and llh with different output parameters

[dweindl]

It would be convenient if one could easily recompute things like ReturnData::y and ReturnData::llh for different output parameters. One application would be hierarchical optimization, where we want to recompute those values with e.g. new scaling parameters or standard deviations without repeating the simulation.

Would be best to do that only after #11

It might make sense to split the currently huge Model into something like DynamicModel and OutputModel.