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. 
If we want a print to show the progress, then we should add `disp` or `silent` as keyword to turn it off