Option to save network state to json string

[TristanM-TD]

It is already possible to save the network state (e.g. using network.save(path) or network.export(path)), however it would also be useful to "just" get the json string representation as opposed to the files, which in turn can be stored in a variable or written to a database.

Ideally, one would then also be able to use the same string representation to reload the state of the network or initialise the solver.