MNLLocationChoiceModelGroup

Adds an `MNLLocationChoiceModelGroup` class analogous to `RegressionModelGroup` for managing models segmented across a population (in this case it's the choosers table that is segmented).

If @janowicz and @fscottfoti like the look of #32 I can also add a `SegmentedMNLLocationChoiceModel` here to reduce some of the work of managing several models that have many of the same parameters.