change network configuration in the training config as well as in the .pth file for the model

This is part of #68 (closed) and implements the format suggested in that issue.

It also changes yet again the format of the .pth file output by dnadna train.

Where previously it had a 'net_params' key, it now just has a 'network' key in the same format as in the config file, containing both the network name, and the parameters it should be initialized with, all in one place.