BUG: LikelihoodModelResults.conf_int use df_resid_inference closes #1903
Merge pull request #1914 from josef-pkt/bug_pvalues_use_t_1903

BUG: LikelihoodModelResults.pvalues use df_resid_inference
Add option to set covariance clipping method, improve reporting of linalg errors
Improvements to params_sensitivity
Don't alter the user's model instance
Incorrect initialization of cov_adjust
Fix #1877 relating to missing data handling in GEE
Don't test for missing with formulas
Add failing test t_est_missing_formula
REF/ENH Merge branch 'gee_sensitivity3_rebased' closes #1840
BUG: Don't use negative indexing with k_ar == 0. Closes #1858.
Merge pull request #1859 from jseabold/arimax-forecast-bug

BUG: Don't use negative indexing with k_ar == 0. Closes #1858.
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.