Mice

This is a WIP PR. I have implemented a statsmodels class that receives a dataset with missing values and imputes the missing values using chained equations. It can then fit an analysis model to the fully imputed data. The code gives sensible results for a real dataset but has not been rigorously tested against existing software packages. 
`return_class` is not a good name, 
sometimes we use `return_xxx` as a boolean that we should return extra results.

I haven't figured out what `return_class` should be.   something like `rvs_class` or, `random_class` might be more informative/explicit/descriptive