Remove RG dependency

[jonschwenk]

Involves copying rivgraph.im_utils.regionprops() and .crop_binary_coords(). The OpenCV dependence might be removable if https://scikit-image.org/docs/dev/api/skimage.measure.html#skimage.measure.find_contours provides the exact functionality of OpenCV's findContours. However, this is a heavily-used operation so lots of testing would need to be done to ensure that it's a 1:1 swap. No need to remove OpenCV, just throwing that bit in.

[tzussman]

Making new issue for OpenCB dependency