Network without layers

New modules: network, nn_node, link which control the network simplifying the structure removing the layer concept (similar to digraph: https://erlang.org/doc/man/digraph.html).

Transformation module not needed as the network concept is created. A network can be modified multiple times without the need of saving each time in mnesia.

Link module is extracted from neurons, so to modify a link, it is not needed to write two parallel transactions to the neuron input and output. Only one to the network.

Elements module merged into cortex, neuron and network.