Bug gee cov type 1906 rebased

rebased version of #1916, makes #1916 obsolete except for comments

one commit was heavily edited during merge conflict with attribute handling in subclasses.
In contrast to Kerby's previous version, I add/copy now all attributes in the subclasses through an argument to the `results.__init__`.   Kerby assigned the attributes after creating the instance, which is too late for the attributes that are required to be available in `__init__`.

TODO: now subclasses can also be added to the unit test to make sure `cov_params_default` is correctly set.  Done for the `cov_params_default` test, not added to generic tests in `base`

I renamed cov_type and the cov_xxx in a not backwards compatible way.
(I needed to fix my noteboook for this two renamings.)

I renamed `resid_xxx` and `sensitivity_params` to use postfix qualifiers, but I kept the old names as alias. I didn't adjust the code to use the new names.

**Summary of PR**

- fixes cov_type handling  #1906  (original PR for this #1916 )
- fix NominalGEE to work with pandas DataFrames and Series #1931
- fix predict  #1919 
- add results wrapper: `fit` now return wrapped instance, with pandas for the main results #1904
- add df_resid so that `t_test` works  #1918
- rename covariance_type -> cov_type
- rename `resid_xxx` and `sensitivity_params`, with aliased old names
- more unit tests, all changes are covered

open 
- remove alias names
- make cov_type calculations lazy  #1910
- review arguments transfer from fit to results, after or if cov_types are calculated through results.
- verified or regression unit tests for cov_robust_bc`