Adding stronger support for 2D neural networks in DeepNeuro, callbacks, GANs, and convenience features.

[AnnaBeers]

This is a large pull request containing many features meant to address some lingering feature requests in DeepNeuro, including

• (More robust) support for 2D image processing
• Greatly expanded Tensorflow model framework
• Boilerplate code for GAN models
• Expanding upon "dn_ops" customized DeepNeuro tensors that can take 2D or 3D forms depending on parameters.
• A tentative framework for custom callbacks building on top of Keras' callbacks.
• Built-in utilities for visualizing data.
• Addressed problems with saving out to multiple formats.
• Various convenience changes to improve usability in pipelines.