Feature request: Add support for metadata in saved networks

[phschafft]

When saving a network there seems currently be no way to store any kind of metadata along side it. This means that for any real world software you need to provide additional information alongside the network, often in a second file.

I would like to suggest to adding support for a key-value string store, ideally one global and one per input and output neuron. This would allow things like assigning names and IDs to each neuron so software can load and auto-connect them (e.g. connect a given input to a specific sensor). Also human readable descriptions could be stored.

A good implementation would likely define some well-known keys plus define how implementation specific keys should be structured.

Such an implementation does not require any kind of performance optimisation as it should only be read/written once during network load/store times.

If consensus that this is useful is reached I could prepare some code for discussion.