Resolve RAP-784 "Image cube step"

This MR adds a new imaging operation class (ImageNormalize) and script (make_image_cube.py) that will be used in flux-scale normalization (see RAP-784). To make adapting the existing imaging workflow easier (and to avoid duplicating a lot of code), some refactoring of the other imaging classes was done.

Note that the new operation is not complete (a script for determining the normalization factors is still needed), and therefore it is not yet enabled in this MR.