Implement progressive U-Net

[kaczmarj]

Progressive training has shown promising results in GANs (https://arxiv.org/pdf/1710.10196.pdf). Let's try this out with a U-Net. The general idea would be to begin training on low-resolution scans and progressively add mirrored layers to the model. We would probably have to start close to the latent space of the U-Net, because that is where images are at their lowest resolution. Layers would be added on either side of the latent space, until we reach our target resolution.