robust covariance, cov_type in fit

integrating robust covariances into the model fit method, see issue #1418 
summary issue for robust covariances is #1158

continuing after PR #1867
now starting with discrete models to build the generic version



two possible problems using a standalone function, e.g.
`get_robustcov_results(res_olsg_._results, cov_type='HC1', use_self=True)`

 - cache already has content that doesn't get overwritten
 - we cannot call the standalone function with the wrapper. We need access and change the underlying Results instance.

`use_self=True` is only intended for use in a results.__init__ when we can make sure we haven't used the cache yet, and internally we only have the results instance and not the wrapper.
do we use default for maxlags in HAC?