WIP: GradOutHook, MergedComposite and multi-hooks

[chr5tphr]

• support the use of multiple hooks by checking for tuples form Composites' module_map and registering all of them (multiple Hook instances only overwrite the results of another)
• add HookBase as another layer of abstraction, above both Hook (which hooks over one full module) and GradOutHook (which hooks only the output of a function)
• add GradOutHook, which can modify the gradient at the output of a module
• add MultiComposite as a parent to MixedComposite
• add MergedComposite, inheriting from MultiComposite, which merges rules returned from composites for a multi-hook setting rather than using the first hook that applies

Todo:

• finish functionality
• finish docstrings
• extend documentation (How-To etc.)
• write tests