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.