Changed the default behaviour when multiple croppers match

When images have mixed annotations (some with only bbx, some with bbx and eyes), you cannot assure that always exactly one cropper matches from the list of croppers. Currently, when this happens an exception is raised. I would propose to use the first valid cropper instead, so that the order of the croppers matter, and one can provide a standard cropper and a fallback solution when the standard does not work.