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)
REF/TST: sandwich arrays add score_obs, add GLM-Poisson tests
ENH: add cov_type to GLM.fit, with tests for family Poisson
REF/ENH: add get_robustcov_results generically to LikelihoodModel/Results
   NegativeBinomial as test case, needs cleanup for code duplication.
REF/TST generic cov_type, TST compare GLM Logit, GLM OLS (this fails)
REF: GLM change default scale in score_obs (tests pass, possibly wrong, see PR comment)
BUG: fix used hessian in sandwich_covariance, with GLM test cases
BUG: genmod links, CDFLink, probit deriv2 use approx_fprime (not complex)
ENH: cov_kwds add generic 'scaling_factor' option to match Stata
Merge pull request #1870 from josef-pkt/ENH_covtype_discrete

ENH: robust covariance, cov_type in fit, in base, discrete and GLM