Feat: Add option for overlay in IndexTracker
This adds the option to have a second, overlayed 3D array display on top of the "core" array in IndexTracker.
As is this allows only one overlay, in theory one could generalize this to N layers? I am unsure how worthwhile that is (or even this merge request).
It has been useful on my end to overlay segmentations on a CT image and/or dose grid