BUG/ENH fix llnull, extra kwds to recreate model

This adds `_init_keys` to `base.Model` and fixes `llnul` in discrete models

includes one test precision adjustment for `TestProbitCG.test_conf_int` (use relative precision)

llnull
BUG #1221 missing `exposure` in llnul of countmodels
BUG #1608 missing `loglike_method` in llnul of NegativeBinomial

recreating models see #1093

no test yet for nb1 versus nb2 with exposure