check bounding box; fixed image cache key

[de-code]

part of https://github.com/elifesciences/sciencebeam-issues/issues/190

Main changes:

• It is now calculating a score of Structural Similarity (using skimage)
• The score is now used as the primary sort key
• Bounding boxes can be visualized using --output-annotated-images-path
• Fixed cache issue that was caused by using id of objects as a cache key, for objects that may get freed with another object being located at the same memory address (id)