Support editing models inside of layers

[Octogonapus]

TensorFlow v1.15.0 added layers that can contain entire models. I have triaged this issue enough so that Axon can load/save models using this feature, but Axon should be able to edit the layers in these models like it can with TF v1.14.0.

This recursive structure presents a bit of a problem for the code generator and for the model editor UI. I think the best solution is to flatten the layers like TF < 1.15 did. This can happen at the HDF5 parsing layer.

Axon still works fine with v1.14 and now at least produces correct (although feature-limited) code with v1.15), so I think we should wait to handle this issue until much later on.