probability and choice modes for DCM

Note that none of this functionality is implemented yet,
this commit introduces the arguments to calls, doc strings,
yaml, and tests.

The options will allow users to specify how they want probabilities calculated
and choices made. This will allow users to choose between
calculating probabilities for all choosers or just one,
and if they want choices made per chooser or for all choosers at once.
Users can also provide their own functions for calculating those things.