Updated viewer widget and added AB test widget

[JLrumberger]

What is the purpose of this PR?

1. Add a viewer for the AB test between PIXIE and nimbus+pixie.
2. Improve the regular viewer widget

How did you implement your changes

1. Added a viewer widget comparable to the one in the video. In addition to the features in the video I added a third option for the cell type "None of the above".
2. I also added a notebook with a mock cell table, that can be used to assess the viewer widget.
3. Save and next button saves the annotations to $output_dir/annotations.csv and jumps to the next cell. If a user wants to stop and continue an annotation run, the input cell_df should be the annotations.csv for the continuation.
4. Added instance segmentation overlay (comparable to the feature in the video) to the regular viewer widget.

cell_analysis.mp4

Remaining issues
None? Noah, do you want to have additional features?

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[ngreenwald]

Looks great! I think we'll know if any features are missing once we try it out, for now this looks like a great MVP