First cut at adding SubtractOverscan, CorrectGain, and CombineHalves classes.
Split of implementation to functions, and call these in the __call__ function.
As a result, the CorrectGain and CombineHalves classes are almost empty shells.
It may now be more logical and easy to define these inside the pipeline.
Merge pull request #4 from mhvk/gmos/basics

First cut at adding SubtractOverscan, CorrectGain, and CombineHalves classes