Undecided trial support

[covertg]

Support for undecided trials is currently not fully baked into model fitting and other parts of the codebase. E.g.: model loss functions (can't combine discrete and continuous likelihood); Sample conversion methods (to and from numpy/pandas doesn't have a set representation for undecided trials, c.f. https://pyddm.readthedocs.io/en/stable/cookbook/loss.html#loss-undecided); and so on.

As discussed offline — making this issue as a reminder and to consolidate further discussion of undecided trials.