Choicemodels

- improves documentation and packaging

- prevents an infinite loop in `interative_lottery_choices()` when none of the remaining alternatives can accommodate any of the remaining choosers

- adds a check to the `MergedChoiceTable` constructor to make sure there aren't any column names that overlap between the observations and alternatives tables

- resolves deprecation warnings from older code

- removes `choicemodels.tools.mnl_simulate()` (originally from `urbansim.urbanchoice.mnl`), because this functionality has been fully replaced

- removes `choicemodels.Logit`, which wrapped a StatsModels estimator as proof of concept for MNL and didn't provide much value on its own

- adds a `chooser_batch_size` parameter to `iterative_lottery_choices()`, to support batch simulation for very large datasets

- adds a function `choicemodels.tools.iterative_lottery_choices()` for simulation of choices where the alternatives have limited capacity and choosers have varying probability distributions over the alternatives

- in `MergedChoiceTable`, empty choosers or alternatives now produces an empty choice table (rather than an exception)

- adds support for multiple tables of interaction terms in `MergedChoiceTable`