Add param_keywords to ParamOverrides and allow ClassSelector to also select classes rather than instances

This pull request consists of two separate modifications:
1. In ParamOverrides introduce a new method param_keywords() which returns all keywords except of those in extra_keywords. This is helpful in case one wants to pass these keywords to the __init__ method of the superclass, because passing all keywords leads to a warning.

2. In ClassSelector allow to actually select (sub-)classes rather than only instances of classes.