REF: More general checking of MRO for Multinomial.

Looking at re-using some multinomial code for ordinal models (though maybe not super efficient memory-wise). Relax this check.
> Why?

For exactly the same reason that you are changing this now. 
If the generic, super class code does type checking, then we cannot extend it or apply it to new models without changing the generic, super class code.
A flag or property check only requires that the model classes define the flag or property.