Improve bridging top down and confidence (BREAKING CHANGE)

[peabody124]

I'm planning to make two changes to how we handle the bridging keypoints that would be a breaking change and require recomputing lots of things:

• we should use the precomputed bounding boxes and feed them into the bridging wrapper, rather than using the IOU with the automatically determined bounding box
• we should tune how we convert the noise into confidence intervals. currently the sigmoid ends up too sharp and is mostly a binary signal.

[ktshah04]

The first bullet point from this issue is addressed in #43