Removed get_loaded_image_count from model (not used).
ValidationModel now has an ImageManager instead of a 
DownloadManager.  The ImageManager has a DownloadManager and the
downloaded items.  It can be expanded to manage both singlets and 
triplets.
ImageCutoutDownload.download_cutout now performs the operation
previously done by DownloadRequest.execute.
Renamed SourceSnapshot to SourceCutout.