Add mechanism for creating networks from plaintext or similar

[lubbersnick]

See related #94 .

It would be preferred if we had a mechanism to save/load models in a way that is safe with respect to pickling - that is, can reliably be expected not to contain arbitrary python objects.

A simple thing we can do, which I have done on a script-based level, is record model creation function in a python file, and other data in json. It may be difficult to serialize the full model construction to a non-python format because of the graph-based model creation system, which allows arguments for different nodes, and even allows nodes to use different modules behind the scenes. However, in the typical case building a model can be reduced to a user task requiring only a handful of lines of code. While such an artefact does techncially contain the possibility of arbitrary code, it can be readily and quickly inspected by a user. The network hyperparameters went to a json file, and the weights to another json. Various related format options are possible. The main point would be to get these into a re-usable set of functions in the library.