First cut at adding SubtractOverscan, CorrectGain, and CombineHalves classes

@wkerzend: I actually did manage to get some of the coding done on the plane! It reproduces mostly what I had before, but with classes. A difference from what we had on the blackboard is that both the bias and data slices are relative to the slices in the image headers. By also taking into account the read-out direction, this makes it possible to just give slices that hold for all the amplifiers. In principle, one could make it more general, but it would not seem necessary.