Remove OpenCV dependency

[jonschwenk]

I just replaced the use of OpenCV's contour finding in rabpro's copy of regionprops. It's not well-tested at this point but I think the use cases should all be simple enough--only thing I'm not sure about are holes in the blobs.

Anyway, assuming it holds, we can replace cv2 functions in RG's regionprops function using the same code as the commit referenced here VeinsOfTheEarth/rabpro#50 points to. There are still some convolutions done by OpenCV in RivGraph that would need to be replaced, but I think there should be drop-ins (or very close) for those in skimage.