Allow cropping by bounding box

[kdmarrett]

--image-offsets coupled with --image-lengths allows you to crop an image to a specific bounding box, for example a bounding box containing only the proofread seeds or connected_sdf from a previous run. This could boost the signal to noise ration due to high saturation in regions outside of the bounding box.

Bug is when passing offsets of x and y, currently only z works.