Keras Support

[MLRichter]

Is your feature request related to a problem? Please describe.
Tensorflow is currently not directly supported by the library.
Since tensorflow allows to fairly easy export the layout of the model via json and yaml, it should be fairly straightforward to implement and encoder that can transform keras-model into a graph of rfa_toolbox.graph.EnrichedNetworkNode objects.

Describe the solution you'd like
Externally, a function that work analog to create_graph_from_pytorch_model()

Additional context
Behind the scenes, the encoder could operate using the json and yaml export functions of keras-models to extract all necessary information to extract the graph.

[MLRichter]

Implemented