Extract Background calculation from BackgroundSubtraction as standalone operator

[jonnew]

The BackgroundSubtraction operator performs two potentially useful calculations:

1. Calculate the "Background" of a video using an selected of method and adaptation rate
2. Perform image segmentation by subtracting the background from the most recent image in the video stream and thresholding result

bonsai/Bonsai.Vision/BackgroundSubtraction.cs

Line 137 in b253870

CV.Threshold(difference, output, ThresholdValue, 255, ThresholdType);

I think the first of these operations is a useful operator on its own. Providing a sequence representing the background of a video could be useful for downstream operations that are not just threshold-based image segmentation (e.g. for dF/F calculations in calcium imaging data).