Improvements to decorator.classmod

[timbernat]

Variety of planned enhancedments and implementation chages to decorators.classmod, including:

• add class decorator which makes certain attributes immutable (based on some of the approaches presented here)
• Reimplement register_subclasses decorator using init_subclass
• Add mechanism for injecting typehints into register_abstract_class_attrs()
• Implement combined decorator for register_subclasses() and register_abstract_class_attrs() for the special case where only one such attr must be registered