Merge pull request #1914 from josef-pkt/bug_pvalues_use_t_1903

BUG: LikelihoodModelResults.pvalues use df_resid_inference
BUG: GEE partial cleanup of cov_type, see #1906
BUG/TST: GEE fix predict closes #1919 and fix conf_int for cov_params_default
add GEE to generic test, and adjust those (missing _results, summary2)
REF: GEE summary remove covariance_type argument (not possible)