ENH: add use_t to LikelihoodModelResults pvalues and confint closes #1812
BUG: GLMResults.summary fix use_t closes #1829
BUG: summary2 use use_t from results closes #1828
REF RLM, discrete summary, use_t from results
REF: add use_t as class attribute
Allow group variable to be passed as variable name when using formula
Merge pull request #1830 from josef-pkt/use-t_issue1812

BUG/ENH Use t in LikelihoodModelResults
Fixed bug in score test
Simplified test code
Remove unwanted print statement
Added test comparing score test to Wald test
Improvements to starting value handling
Merge pull request #1839 from kshedden/gee_score

BUG: GEE score
Merge pull request #1837 from kshedden/mixedlm_formula

ENH: Allow group variable to be passed as variable name when using formula
TST: anova with `-1` noconstant, add tests
REF: anova_lm: use diff(df_resid)
Merge pull request #1770 from josef-pkt/anova_noconstant_1213

TST/REF: anova with `-1` noconstant, add tests
BUG/ENH: fix and improve constant detection
REF: _handle_constant, check implicit in all cases without non-zero constant
REF: remove try ...except (no other changes)