Expand OPN/CPN to n-choice models

Right now, OPN/CPN networks can run into problems for some of the models that we can simulate via `ssm-simulators`.

Two steps:

- [ ] we need to take stock of any such models that may create problems (close via #27 )
- [ ] we need to do the actual work to generalize out networks to deal with n-choice scenarios (first problem: `bcelogit` loss)

Start by taking stock of models that can be simulated via ssm-simulators and may cause problems in OPN/CPN networks. Then investigate how to generalize the networks for n-choice scenarios, beginning with the bcelogit loss; done means the identified models run without these issues.

Written by the indexing model from the issue text.