Remove single chooser restriction from DCM

We'd hardcoded that when doing prediction a DCM would
select only the first chooser from the choosers table
in order to get a PDF used when assigning choosers
to alternatives.

I've removed that so now all choosers go into making the
interaction dataset and probabilities come back calculated
one per chooser across all alternatives. Obviously we'll need
to figure out a way to make this manageable for doing LCMs.

I think the unit_choice method still needs work, bits and pieces
of the code seem assume that there will be only one set of
probabilities for all choosers, not probabilies per chooser.