strategy to deal with high-resolution images

[dalessioluca]

the feature map can be from some lower_level.
the unet can go down all the way to 1x1 so that I can extract the background easily.
the sliding_window can be smaller so that only 3, 4 cells are in it (reducing N_BOX will be computationally convenient)
Unet can use dilation to deal with large images.....