BUG GEE cov type

(need to rebase, there is one commit from another PR)  DONE

see #1906
first part of cleaning up the covariance_type for GEE 

`cov_params_default` is now correctly chosen
incomplete refactoring
- bse/cov_params has unit tests in test_gee
- the consistency of tvalues, pvalues, conf_int, t_test and wald_test are tested in base test_generic_methods,  with monkey patched `df_resid`

This finishes the minimal bug fix refactoring, except for 
- missing `df_resid`
- subclasses Ordinal and Nominal don't use all covariance types

TODO:
covariance_type -> cov_type

optional for now:
remove `GEEResults.conf_int` method

more refactoring
move covmat calculation to being lazy in Results   (`_get_robustcov_results` pattern)
(this would also require fewer extra keywords from `fit` to `Results.__init__`