Allow image data to be shown with colormaps

[almarklein]

The image data is now greyscale. Being able to show it in e.g. Viridis would be nice. We should consider making it possible to create a colorbar in this case.

There are different ways to implement this. Ranging from leveraging numb, scikit-image, or Pillow. One very interesting approach is to produce paletted PNG's, with the palette being the colormap. This can be done via e.g. Pillow or pypng.