REF: Use complex step derivatives
STY: x**-1 -> 1/x
REF: Micro-optimization don't use self.
ENH: Fix fit arguments. Add correct aic and bic for nb1/2
TST: Add correct test values from stata
BUG: Fix alpha/lnalpha bse for nb1
TST/STY: Some pep-8. Lower precision for conf_int for alpha.
TST: Add conf_int results from stata
TST: Adjust precision for test