Merge pull request #1867 from josef-pkt/REF_covtype_fit

ENH/REF add covtype choice to fit method
ENH: start with generic robust covariance, copy-paste from linear_model
ENH: base get_robustcov_results: add white_simple as HC (remove obsolete line)
TST: discrete test_sandwich_cov, add Poisson case for new get_robustcov_results
ENH Poisson.fit  add cov_type option (first specific version)
REF: RegressionModel.fit explicit uxe_t keyword (TODO: remove old usage)