Remove "region of interest"

DetectorMask and RegionOfInterest provide two different, indendent mechanism for closely related purposes (DetectorMask is more generic, supporting polygonal or ellipsoidal shapes; RegionOfInterest is always a rectangle). Merging them will considerably simplify the code. Right now, RegionOfInterest is in my way as I am simplifying the ICoordSystem classes.

Therefore, let's remove RegionOfInterest for good. Later, we will expand the DetectorMask machinery so that negative masks can easily be used to select a region of interest.