ENH: add cov_type to Regression model.fit(), first version
DOC: covtype in fit: update example with cluster
BUG: use use_t argument
REF: base Model, new optional attribute df_resid_inference
TST: robustcov add test for fit options and for scale, rsquared
     fix example
Merge pull request #1867 from josef-pkt/REF_covtype_fit

ENH/REF add covtype choice to fit method