Support for recurrent models?

[jacobpennington]

With a few extra tweaks, I think the new arbitrary input/output scheme could support recurrent architectures. Would just need to keep the intermediate DataSet object (i.e. containing all saved outputs) around during fitting, which should probably be done anyway to reduce overhead, and add a way to specify some kind of default if the output doesn't exist (e.g. on first evaluation, if receiving input from a later layer that hasn't been evaluated yet).