Automatic Serialization

[LarsKue]

The goals of this PR are:

• Remove the need to implement from_config and get_config methods for most objects
• Remove the need to save a config object in the constructor of most objects
• Unify the serialization of types, functions, and other objects
• Improve the serialization of functions, especially with respect to numpy

To achieve the above goals, this PR introduces:

• A custom serialize and deserialize function that wrap around the keras utilities
• A custom serializable decorator that automatically implements from_config and get_config

[stefanradev93]

That's a super nice utility! Which networks are already tested through?

[LarsKue]

It's still in progress, so not functional for all networks yet. But I made changes such that most networks should already be tested.

[vpratz]

This looks really nice! Please take care when removing get_config functions, some attributes might accumulate state that is not automatically serialized by Keras, so already serializing them when __init__ is called might not be sufficient. I'm not totally sure, but I think this might apply for example for ContinuousApproximator.adapter.