Expand OPN/CPN to n-choice models

[AlexanderFengler]

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 Take stock of all models that currently can't be trained #27 )
• we need to do the actual work to generalize out networks to deal with n-choice scenarios (first problem: bcelogit loss)